It ain't about age, but maturity. Give it to them when they appreciate what it is and have the responsibility to not lose their keys. Might not be till they're 30, for all I know. Hopefully they are raised to be more mature and responsible earlier than that, but you know her parents way better than any of us.

18 but prob 25 I'd say

When her frontal lobe is fully formed so around 25

I'd let her have X% on a software wallet to spend learn make mistakes but keep Y% a much larger percent on a hardware device and give to her on whatever birthday you deem. Monetary lessons were learned

But definitely once old enough should have Access to a small percentage of And then she could be paid an allowance with different percentages, going to different wallets etc

Congrats on the new niece!