Sorry I forgot you wanted to see if the person reached the age of 65 before B1.
Well you have to ask a question, what determines before B1 what point in time do we use. I used today’s date Now(), as follows:

=IF(DATEDIF(A1,NOW(), "Y")=65,TRUE,DATEDIF(A1,B1,"Y"))

This formula finds the age of the person from A1 to now, if the person has reached 65 the formula returns True, if not it finds the age between A1 & B1 and returns the age between the two dates. This way if the person reaches 65 by today you know it, if not you know how old the person is based on the two dated A1-B1.

Hope this helps.