My first instinct was to say no, because 99.9% of the time I'm reading manuscript submissions, I don't know the age of the author. Even if the author gives some hints in the cover letter (for example, by saying that he or she is a retired librarian), it wouldn't affect my judgement, because I don't generally read the cover letter before reading the manuscript. It's hard to say for sure, but I'd like to think that even if I did know an author's age, it wouldn't affect me either way, because it's the writing that I fall in love with.
However, the one time I have seen the author's age make a difference is in an auction situation when we're deciding whether we want to compete with other publishers for a project. When we make an aggressive offer on a project, we're looking to make an investment in the author's career--generally, we're not looking to publishing "one-off" books. And if we feel that the author may not have more than one or two books in them, we might not think it's worth the investment. However, I can't imagine that this would come into play unless an author was over, say, 75 or so.
An author's age may also make a difference in terms of Publicity. The publicist is looking to see how "media-genic" an author might be, and so, yes--I think in some cases, if an author is young (and attractive!), this might be taken into account. I think especially for teen readers, they like to think that the author is young and hip and cool, and in the age of social networking, they have easy access to information about the author in some cases. Then again, if an author is older, it may actually be a plus for publicity. For example, author Millard Kaufman published his first novel at age 90, and his age made for a great publicity hook, just as Christopher Paolini's young age did, too.
It was interesting that the email specified older books, because I don't think ageism affects picture books at all--in fact, having a "grandmotherly" or "grandfatherly"-aged author might actually be an advantage.
If you're an older author and aren't having luck getting a book published, I don't think you should or can blame ageism. Publishing is a difficult, competitive industry no matter what your age. And I think older authors have the advantage of having more of a wealth of life experiences to draw on, which gives them an advantage over younger authors. You need to focus on the positives, know your weaknesses, and keep working to improve. I do think that in some cases, older authors might have a disadvantage in writing contemporary YA novels, because it's hard to write authentic teen dialogue, and the farther away you are from your teen years, the more foreign their language may seem. However a 15-year-old or a 30-year-old can be just as guilty of writing inauthentic dialogue as much as a 60-year-old. And no matter their age, authors can find opportunities to be around teens, observe them, listen to their language.
In my opinion, some of us may be guilty of ageism in Publishing, but I honestly don't think it comes into play very often. But I wonder if any of you feel differently or have had personal experiences to share regarding this issue. Because if even a little ageism exists, we should move toward eliminating it. Please share!