Without roles, an 18- or 19-year-old at school is read as the head's partner when the head is less than 16 years older #2289

What happens

When a household is entered without roles (is_claimant_or_partner) or parent flags (is_parent), is_claimant_or_partner reads an 18- or 19-year-old in non-advanced education as the head's partner if the head is less than 16 years older. The same happens under a flagged-parent rule for a grandparent's flagged daughter.

On main (9475c95), 2026-27, a head aged 33 with an 18-year-old in upper secondary education and no flags: is_claimant_or_partner is [true, true] and is_couple is true.

The steps:

1. is_hbai_dependent_child counts an 18- or 19-year-old in education as a dependant only if someone in the benefit unit is flagged is_parent. With no flag, they are an HBAI adult.
2. is_claimant_or_partner presumes a member is the claimant's child only when they are under 20 and at least 16 years younger (household.demographic.benefit_unit.presumed_child). At 15 years apart, the 18-year-old becomes the partner.

Why it matters

The model then treats the two as a couple everywhere: couple rates, joint means tests, and no child amounts for the student. In law a person can be responsible for a young person who lives with them whatever their relationship (for Child Benefit, SSCBA 1992 s.143(1)(a)), so a sibling, aunt or young step-parent caring for an 18-year-old at school gets the child amounts and single rates.

Until #2285, Child Benefit, CTC and the Pension Credit child addition still counted such a "partner" as a qualifying young person, so the family was a couple and also got child amounts for one of the couple. #2285 stops that double count, and pins this case in claimant_not_own_child_or_qualifying_young_person.yaml (Cases 9 to 11): flagging the head is_parent, or supplying the roles, gives the right answer.

Households affected (traced through the formulas; the first was run):

• head 33, student 18, no flags: read as a couple;
• head 25 caring for a sibling aged 18 at school, no flags: read as a couple;
• an unflagged grandparent aged 60, a flagged daughter aged 19 at school and her baby: the daughter is read as the grandparent's partner (the docstring's "exactly one other HBAI adult is flagged" rule).

This only affects inputs without roles. Datasets that supply is_claimant_or_partner are untouched.

Possible change

Presume that a member aged 18 or 19 who is not the head and is in non-advanced education or approved training is the head's dependant, not partner, when the head is older by some smaller gap than 16 years.

This is a choice about calculator defaults, like #2082 (20+ years younger at any age), so it needs the same evidence and the same ruling: the share of co-resident pairs, where the younger is 18 or 19 and in non-advanced education, that the FRS records as couples, by age gap.

Found in review of #2285.