Firstly, several people who I know are OAPs have come to my classes and have received book deals from mainstream publishers. I don't know exactly how old they are, but definitely beyond retirement age (I believe one lady is well into her 70s). I've also heard from other first time 65+ authors who have recently had book deals.
So age doesn't appear to matter - if you have written a good book. Where I think age may count against you is if your book is borderline good, so you need to do all you can to counteract this. The relevant factors are:
Career span - how many more books are you going to produce? Money is made from authors writing more than one book, so if you're older you need to stress that you've got more than one book in you. One of my former students has had 9 books published and is busy writing number 10.
Marketability - yes, photogenic is useful, but so is a good story. If you're older, you should have some interesting life experience that could be used for publicity (beyond OAP writes book, which has been done several times before). Think about what you could use from your life story to sell your book.
Self promotion - like it or not (and most of us don't) authors are expected to do a lot of self promotion. It's becoming a factor that sways decisions. No one knows how old you are on-line, and there's nothing to stop you blogging/Twittering/building a social media platform. If you say you're too old to do all that, then you may have to accept you're too old to get published. If, on the other hand, you're prepared to throw yourself into promotion, then you're not to old. It's your attitude that counts here, not your physical age.
Finally, why would you (or anyone else) mention your age on a submission letter? It never occurred to me to give my age. I don't think either my agent or my editor know it now, and I don't know theirs. It seems especially daft if you think it might deter an agent. Why do it? Yes, if they like your manuscript so much they want to meet up, then you'll have to reveal yourself, but by that stage they'll be pretty keen and it's unlikely to be an issue.
I think the real answer is to write a good book that no one can turn down, and then you can be any age you choose.