I am not sure if I care about detection. They will obviously shut down your account if you're too young, but they don't have any legal demand to shut down your account if you're too old, so they probably won't bother.

Most of the places where I see these things are not services like Dropbox where I'm a customer, but for things like alcohol marketing websites. "To view our collection, you must be over 21. Type your age to verify." The law requires them to ban me for being under 21, but doesn't require them to ban me for being "too old", so they probably don't bother. (I imagine the hot topic at meetings is when they provide a year dropdown, what's the oldest year they should add. If the world's oldest person wants to visit their website, that should be allowed.)