[MNT] Capitalized age heuristics

[rmanaem]

Closes #561

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

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[surchs]

Test changes look good! 🧑‍🍳

Sidecomment: not sure the specific controlled term / the stuff you just changed has any effect in the tests. i.e. the fact that you made the tests align with the hardcoded values in the store is very important, but nothing would break if you had forgotten that, right? My point is: it would be nice if we could make it so something does break if we forget - so that we don't forget.