is_apprentice sets both the apprentice minimum wage and the s.9B employer NICs relief #2683

is_apprentice now has two readers that mean different things.

• minimum_wage and minimum_wage_category give the apprentice rate whenever the flag is true. The rate applies only to an apprentice under 19 or in the first 12 months of the apprenticeship (National Minimum Wage Regulations 2015, reg. 5), as apprentice.yaml itself notes.
• secondary_class_1_threshold (Zero-rate employer NI for under-21s and apprentices under 25 #2505) reads it as a relevant apprentice under 25 (SSCBA 1992 s.9B), in any year of the apprenticeship.

PolicyEngine/microcosm#1373 sets the flag for a share of employees aged 21 to 24, so the s.9B relief reaches them. Those people then also get the apprentice minimum wage: £7.55 an hour in 2025-26 and £8.00 from April 2026, against £12.21 and £12.71.

minimum_wage feeds three income tests that ask for earnings of at least 16 hours a week at the person's minimum wage:

• Tax-Free Childcare;
• extended childcare;
• the Childcare Offer for Wales.

On 2.162.3, a 22-year-old parent earning £8,000 in 2026 fails all three without the flag and passes all three with it. On the Microcosm spine very few parents are affected, but the two readings should still be separate.

One option is a person input, is_in_first_year_of_apprenticeship. minimum_wage would then apply the apprentice rate only when is_apprentice holds and the person is under 19 or that input is true.

• The input's default decides whether a household-calculator user who sets is_apprentice keeps today's result: True keeps it, False doesn't.
• Microcosm would write the input explicitly either way. DfE's starts against its apprentices in learning give a first-year share.

The UC minimum income floor and the benefit cap earnings exception read minimum_wage.non_apprentice directly, so they are unaffected.

Related: #2505, PolicyEngine/microcosm#1372, PolicyEngine/microcosm#1373.