There are plenty of reasons why your dad might not be interested in dating that woman, women like her, or even any women. Maybe there was something about her he didn't like. The mismatch in ages is a very valid concern: people of similar ages are more likely to have similar life experiences and therefore similar outlooks on life. Or maybe he just doesn't want to date again. Maybe right now; maybe never.
All of those are valid. Just like you have to be yourself, and you need others around you to respect that, he needs to be himself, however he defines that, and those around him need to respect that.
It's okay to joke around a little bit about a situation like that, but as soon as he shows that he is not comfortable, it's time to back off.