Lower pension/SS split threshold from 60 to 55 (#935)

Closes #933.

Colorado allows a $20K pension subtraction per filer aged 55-64.
PR #925 split pension/SS only when both spouses were ≥60, leaving
55-59 mixed-age couples allocating the full pension to one spouse
and losing the per-person CO subtraction on the other.

Drop the threshold to 55 (the lowest practical state age gate).
Higher-threshold states (DE 60, GA 62, MD 65) are unaffected: a
55-59 split doesn't create a false exclusion because those filers
fail the higher state age gates anyway.

3K eCPS 2025 sample: identical aggregate match rates (no regression).
Per-record: #933 CO joint, page=58/sage=56, $65K pensions now matches
TaxAct exactly (siitax $1,224.24 vs prior $2,104.24; $40K subtraction
vs prior $20K).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: PavelMakarchuk

changelog.d/lower-pension-split-threshold.fixed.md

@@ -0,0 +1 @@
+Lower the pension/SS age-aware split threshold from 60 to 55 to match Colorado's 55+ pension subtraction (so mixed-age couples ages 55-59 each claim the state per-person exclusion). Higher-threshold states (DE 60, GA 62, MD 65) are unaffected.

policyengine_taxsim/runners/policyengine_runner.py

@@ -195,16 +195,19 @@ def _initialize_dataset_structure(self) -> dict:
         }
     )
 
-    # Pension and Social Security income are split only when both spouses
-    # meet the state-level age threshold (60, the lowest common threshold
-    # across states such as DE). In mixed-age households (e.g. spouse under
-    # 60 while filer is elderly), the income stays with the primary filer so
-    # age-based state exclusions aren't lost on the younger spouse's
-    # incorrectly-allocated share.
+    # Pension and Social Security income are split between spouses only
+    # when both meet the state-level age threshold (55, the lowest common
+    # threshold across states such as CO whose pension subtraction
+    # qualifies filers 55+). In mixed-age households the income stays
+    # with the older spouse so age-based state exclusions aren't lost on
+    # the younger spouse's incorrectly-allocated share. Higher-threshold
+    # states (DE 60, GA 62, MD 65) are unaffected: 55-59 splits don't
+    # create false exclusions because the filers fail those states' age
+    # gates anyway.
     _AGE_GATED_FIELDS = frozenset(
         {"taxable_private_pension_income", "social_security_retirement"}
     )
-    _AGE_GATED_SPLIT_AGE = 60
+    _AGE_GATED_SPLIT_AGE = 55
 
     @staticmethod
     def _make_primary_split(source_field):

tests/test_spouse_income_splitting.py

@@ -107,6 +107,18 @@ def test_pension_splits_when_both_spouses_are_60_plus():
     np.testing.assert_allclose(values, [20000.0, 20000.0])
 
 
+def test_pension_splits_when_both_spouses_are_55_plus():
+    """Pension: when both spouses ≥ 55 (CO's threshold), allocate 50/50
+    so each spouse claims the per-person CO pension subtraction. See
+    taxsim issue #933: CO joint, page=58, sage=56 — both qualify for
+    CO's 55-64 $20K subtraction. Pre-fix PE put pension on primary
+    only, giving $20K total subtraction; correct behavior splits 50/50
+    so each spouse claims $20K = $40K total (matching TaxAct)."""
+    df = pd.DataFrame([_base_mfj_record(page=58, sage=56, pensions=40000)])
+    values = _run_allocation(df, "taxable_private_pension_income")
+    np.testing.assert_allclose(values, [20000.0, 20000.0])
+
+
 def test_pension_goes_to_primary_when_primary_is_older():
     """Pension: mixed-age, primary older — keep full pension on primary
     so they claim the per-person elderly exclusion. Splitting 50/50 would