If you want privacy you need to fuzz the transition. Many platforms support that today. Or you can create a separate account when you graduate.

But also, knowing someone's birthday without trying it to other information greatly reduces the risk of harm.

Not unless you actually meant 16<x<18 today and >18 tomorrow.

You can be 30 and verify >16 today and >18 tomorrow, obviously without being 18.

which is nothing in comparison to leaking all of personal information

you can also introduce some jitter like changing age range only once a week/month/year for everyone

Birthday, zip code and gender is enough to uniquely identify most Americans.

Of course you can still break the law. People are complaining that it's now illegal to do what you suggested

1 it requires DOB

2 it requires DOB to be accurate