Summary

Drop the pension/SS age-aware split threshold from 60 to 55 so that mixed-age couples in the 55-59 range each claim per-person state pension subtractions where the state's age gate is 55+ (CO).

Why

CO allows a $20,000 pension subtraction per filer aged 55-64. PR #925 introduced an age-aware split that fires only when both spouses are 60+. For a CO joint with page=58 / sage=56 (taxsim #933), the full pension stayed on the primary, giving $20K total subtraction; TaxAct gives $40K (= $20K × 2 spouses, each 55+).

Why 55 is safe

• CO threshold = 55+
• DE threshold = 60+ → 55-59 split allocates pension between spouses, but neither qualifies for DE's exclusion regardless. No false credit.
• GA threshold = 62+ → same logic
• MD threshold = 65+ → same logic

Lowering doesn't create false exclusions in higher-threshold states; it only enables the per-person CO exclusion the existing logic missed.

Verification

• 3K eCPS 2025 sample: identical aggregate match rates (no broader regression)
• Per-record CO joint 2025 58page 56sage 20Kpwages 10Kswages 5Kintrec 64Kpensions #933: PE now exact-matches TaxAct ($1,224.24 vs prior $2,104.24; CO subtraction $40K vs prior $20K)
• 18/18 spouse-income-splitting tests pass, including a new test_pension_splits_when_both_spouses_are_55_plus regression case for CO joint 2025 58page 56sage 20Kpwages 10Kswages 5Kintrec 64Kpensions #933

Test plan

• 18 splitting tests pass (1 new regression test for CO joint 2025 58page 56sage 20Kpwages 10Kswages 5Kintrec 64Kpensions #933)
• No aggregate change on 3K eCPS comparison
• CI passes