diff --git a/nibabies/utils/tests/test_bids.py b/nibabies/utils/tests/test_bids.py
new file mode 100644
index 00000000..160421b3
--- /dev/null
+++ b/nibabies/utils/tests/test_bids.py
@@ -0,0 +1,53 @@
+from pathlib import Path
+
+import pytest
+
+from nibabies.utils.bids import _get_age_from_tsv
+
+
+def create_tsv(data: dict, out_file: Path) -> None:
+    import pandas as pd
+
+    pd.DataFrame(data).to_csv(out_file, index=False, sep='\t')
+
+
+def create_sidecar(tsv_file: Path, units) -> None:
+    import json
+
+    out_file = tsv_file.with_suffix('.json')
+    data = {'age': {'Units': units}}
+    out_file.write_text(json.dumps(data))
+
+
+age = {'age': [4, 4, 4]}
+age_weeks = {'age_weeks': [4, 8, 12]}
+age_months = {'age_months': [3, 6, 9]}
+age_years = {'age_years': [1, 1, 2]}
+
+
+@pytest.mark.parametrize(
+    ('idx_col', 'idx_val', 'data', 'sidecar', 'expected'),
+    [
+        ('session_id', 'x1', age, False, None),
+        ('session_id', 'x1', age, 'months', 4),
+        ('session_id', 'x1', age, 'weeks', 1),  # Convert from 4 weeks -> 1 month
+        ('session_id', 'x1', age, ['months', 'weeks'], None),
+        ('session_id', 'x2', age_weeks, False, 2),
+        ('participant_id', 'x1', age_months, False, 3),
+        ('participant_id', 'x3', age_years, False, 24),
+        ('session_id', 'x3', {**age_months, **age}, False, 9),
+        (None, None, age_months, False, 3),
+    ],
+)
+def test_get_age_from_tsv(tmp_path, idx_col, idx_val, data, sidecar, expected):
+    tsv_file = tmp_path / 'test-age-parsing.tsv'
+    base = {}
+    if idx_col is not None:
+        base[idx_col] = ['x1', 'x2', 'x3']
+    create_tsv({**base, **data}, tsv_file)
+
+    if sidecar:
+        create_sidecar(tsv_file, sidecar)
+
+    res = _get_age_from_tsv(tsv_file, idx_col, idx_val)
+    assert res == expected