I try to put birthdate in and it keeps telling me I’m not old enough. I was born in 1977?

The birthdate cannot be changed in your account for security reasons: https://help.yahoo.com/kb/sln27267.html

Try reversing the order of the numbers for the month and day and put a zero in front of the month. United States people and?Europeans can do this differently and Yahoo and YA were  originally programmed by USA people. 

For example, if you were born June 8 1977 that would be 06 08 1977 (American) or 08 06 1977 (rest of the world)

Since you you successfully posted This question on YA and have logged into a Yahoo email account to do that, Yahoo and Verizon thinks you are at least 13 years old. You are either lying or baiting and fishing to spam YA with a new spam bot email account. 

As you are using an account, I don't see the problem.

Try lying about your age. Try 43.