Since it’s about birth dates, shouldn’t it be an astrological amount of people?

That might just make it easier for them to put them all on a list. Use random dates instead, then they have to identify you and confirm you’re not using your real date to put you on that list.

October 15th, 1582 is another one you see often. (Edit: Some date systems use this as 0 because it was the start of the proleptic Gregorian calendar.)

Don’t forget December 31st, 1969 which you’ll see when computers adjust UTC midnight at the Unix epoch to local time in the western hemisphere.

Lawmakers don’t care whether you’re 18 or 56