Presume a much younger member is a flagged parent's child at any age

changelog.d/2039.fixed.md

@@ -0,0 +1 @@
+- A member at least 16 years younger than a claimant flagged as a parent is now presumed to be their child at any age, unless a member under 20 and at least 16 years younger already explains the flag. So a flagged lone parent living only with an adult son or daughter is no longer inferred as a couple. Without any parent flag the under-20 limit stays, because survey childless couples carry no flags.

policyengine_uk/parameters/household/demographic/benefit_unit/presumed_child/age_limit.yaml

@@ -1,4 +1,4 @@
-description: Age below which a benefit-unit member much younger than the head is presumed to be the head's child rather than their partner, when relationships are not supplied. This is a PolicyEngine modelling presumption, not a statutory rule.
+description: Age below which a benefit-unit member much younger than the claimant is presumed to be the claimant's child rather than their partner, when relationships are not supplied. The limit does not apply when the claimant is flagged as a parent and no member under it, at least the minimum age gap younger, explains the flag. This is a PolicyEngine modelling presumption, not a statutory rule.
 values:
   0001-01-01: 20
 metadata:

policyengine_uk/parameters/household/demographic/benefit_unit/presumed_child/minimum_age_gap.yaml

@@ -1,4 +1,4 @@
-description: Minimum age gap below the benefit-unit head at which a younger member is presumed to be the head's child rather than their partner, when relationships are not supplied. This is a PolicyEngine modelling presumption, not a statutory rule.
+description: Minimum age gap below the claimant at which a younger member is presumed to be the claimant's child rather than their partner, when relationships are not supplied. It applies at any age when the claimant is flagged as a parent and no younger member under the presumed child age limit explains the flag, and otherwise only under that limit. This is a PolicyEngine modelling presumption, not a statutory rule.
 values:
   0001-01-01: 16
 metadata:

policyengine_uk/tests/policy/baseline/household/demographic/benefit_unit_structure.yaml

@@ -371,3 +371,276 @@
         members: [head_1, head_2, other]
   output:
     is_claimant_or_partner: [true, true, false]
+
+# Below a claimant flagged as a parent, a member at least 16 years younger is
+# presumed their child at any age, unless a member under 20 and at least 16
+# years younger already explains the flag. Without flags, the under-20 limit
+# stays: survey childless couples carry no flags and include large age gaps.
+
+- name: A flagged lone parent with an adult son is single
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      parent:
+        age: 50
+        is_parent: true
+      son:
+        age: 25
+    benunits:
+      benunit:
+        members: [parent, son]
+    households:
+      household:
+        members: [parent, son]
+  output:
+    is_claimant_or_partner: [true, false]
+    relation_type: SINGLE
+    is_couple: false
+
+- name: Without a parent flag, a 25-year gap is still read as a couple
+  # Documented limitation: indistinguishable from a survey childless couple.
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      older:
+        age: 50
+      younger:
+        age: 25
+    benunits:
+      benunit:
+        members: [older, younger]
+    households:
+      household:
+        members: [older, younger]
+  output:
+    is_claimant_or_partner: [true, true]
+    is_couple: true
+
+- name: A flagged parent keeps an unflagged partner close in age, not the adult son
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      parent:
+        age: 50
+        is_parent: true
+      partner:
+        age: 48
+      son:
+        age: 25
+    benunits:
+      benunit:
+        members: [parent, partner, son]
+    households:
+      household:
+        members: [parent, partner, son]
+  output:
+    is_claimant_or_partner: [true, true, false]
+
+- name: The 16-year gap below a flagged parent is a boundary
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      parent_a:
+        age: 50
+        is_parent: true
+      gap_16:
+        age: 34
+      parent_b:
+        age: 50
+        is_parent: true
+      gap_15:
+        age: 35
+    benunits:
+      unit_a:
+        members: [parent_a, gap_16]
+      unit_b:
+        members: [parent_b, gap_15]
+    households:
+      household_a:
+        members: [parent_a, gap_16]
+      household_b:
+        members: [parent_b, gap_15]
+  output:
+    is_claimant_or_partner: [true, false, true, true]
+
+- name: A flagged grandparent's daughter is read as a partner when a grandchild explains the flag
+  # Documented limitation, as before this rule: the grandchild under 20
+  # explains the grandparent's parent flag, so the under-20 limit stays and the
+  # 45-year-old is the partner. Supply is_claimant_or_partner for such homes.
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      grandparent:
+        age: 70
+        is_parent: true
+      daughter:
+        age: 45
+      grandchild:
+        age: 10
+    benunits:
+      benunit:
+        members: [grandparent, daughter, grandchild]
+    households:
+      household:
+        members: [grandparent, daughter, grandchild]
+  output:
+    is_claimant_or_partner: [true, true, false]
+
+- name: Without flags the under-20 limit still applies at the boundary
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      head_a:
+        age: 40
+      aged_19:
+        age: 19
+      head_b:
+        age: 40
+      aged_20:
+        age: 20
+    benunits:
+      unit_a:
+        members: [head_a, aged_19]
+      unit_b:
+        members: [head_b, aged_20]
+    households:
+      household_a:
+        members: [head_a, aged_19]
+      household_b:
+        members: [head_b, aged_20]
+  output:
+    is_claimant_or_partner: [true, false, true, true]
+
+- name: A young child explains a lone flag, so a large-gap partner stays the partner
+  # Only the 60-year-old is flagged, but the 10-year-old explains the flag, so
+  # the under-20 limit stays and the 42-year-old partner keeps their place.
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      claimant:
+        age: 60
+        is_parent: true
+      partner:
+        age: 42
+        employment_income: 50_000
+      child:
+        age: 10
+    benunits:
+      benunit:
+        members: [claimant, partner, child]
+    households:
+      household:
+        members: [claimant, partner, child]
+  output:
+    is_claimant_or_partner: [true, true, false]
+    is_couple: true
+    is_lone_parent: false
+    uc_earned_income: 31_563.60
+    universal_credit: 0
+
+- name: Flagging both partners keeps the large-gap couple
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      claimant:
+        age: 60
+        is_parent: true
+      partner:
+        age: 42
+        is_parent: true
+        employment_income: 50_000
+      child:
+        age: 10
+    benunits:
+      benunit:
+        members: [claimant, partner, child]
+    households:
+      household:
+        members: [claimant, partner, child]
+  output:
+    is_claimant_or_partner: [true, true, false]
+    is_couple: true
+    uc_earned_income: 31_563.60
+    universal_credit: 0
+
+- name: Supplying claimant and partner roles keeps the large-gap couple
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      claimant:
+        age: 60
+        is_parent: true
+        is_claimant_or_partner: true
+      partner:
+        age: 42
+        employment_income: 50_000
+        is_claimant_or_partner: true
+      child:
+        age: 10
+        is_claimant_or_partner: false
+    benunits:
+      benunit:
+        members: [claimant, partner, child]
+    households:
+      household:
+        members: [claimant, partner, child]
+  output:
+    is_couple: true
+    is_lone_parent: false
+    uc_earned_income: 31_563.60
+    universal_credit: 0
+
+- name: A flagged lone parent with an adult child and a younger child is read as a couple
+  # Documented limitation, as before this rule: the 12-year-old explains the
+  # parent flag, so the 25-year-old (not under 20) is read as the partner.
+  # Supply is_claimant_or_partner to enter this household.
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      parent:
+        age: 50
+        is_parent: true
+      adult_child:
+        age: 25
+      child:
+        age: 12
+    benunits:
+      benunit:
+        members: [parent, adult_child, child]
+    households:
+      household:
+        members: [parent, adult_child, child]
+  output:
+    is_claimant_or_partner: [true, true, false]
+
+- name: An unexplained parent flag on one of a childless large-gap pair makes them single
+  # Trade-off: with no child under 20 to explain it, the flag on the
+  # 60-year-old marks the 42-year-old as their adult child, not their partner.
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      older:
+        age: 60
+        is_parent: true
+      younger:
+        age: 42
+    benunits:
+      benunit:
+        members: [older, younger]
+    households:
+      household:
+        members: [older, younger]
+  output:
+    is_claimant_or_partner: [true, false]
+    is_couple: false