How many sites filter content based on age and really do so selectively based on a birthdate? Most age verifications, even ones that request a date, seem to be just asking the question, "Are you of legal age?"

(I prefer websites just ask that binary question, like this brewery - http://www.newbelgium.com - instead of making me waste time with a date input.)

Your point really does not apply anonymized datasets which are released and used to do statistical analysis on a population.

It may be relevant if you are trying to examine differences immediately before and after your 18th or 21st birthdays, but that would be pretty rare.