I'm sure that ageism
exists in publishing, but it's one of those undeclared aspects of unfairness, hidden by agents and publishers falling over themselves to be politically correct about other areas of bigotry—ethnicity, sexual preference, gender, religion or status in society—is the author a refugee or homeless, for instance.
If you think, for one moment, that publishers and literary agents are unbiased, then try looking at the staff profiles on their websites to see how old the people they employ are. There'll be a few wrinklies, usually the bosses, but the majority of the smiling faces will belong to youngsters. As much as anything, how an author looks is part of marketing: readers are more likely to pick up a novel by an unknown author if they're attractive. No one will ever admit that this is true.
Employment laws theoretically prevent discrimination based on age, as do regulations that stop literary agencies and publishers asking the age of the writer submitting a manuscript, but, as your friend Cyril suspects, they may find ways to stymie applications. I once queried a publisher who got back to me, asking for my writing history, including the dates of any previously published material, which would have indicated that I was at least 60 years-old (or a time traveller).
I once shot myself in the foot with a job application. I went for a position with a housing association, work I'd done before for several years, so I fitted the job profile. A colleague at the community centre, which I'd be helping to manage as a volunteer, suggested that I trim my beard that had grown to winter plumage proportions. I made it look neat, to go with a suit and tie that I barely remembered how to put on, and duly presented myself for interview. I was expecting to be grilled by a young female HR officer, whose name appeared on the correspondence— instead two hippies my age did the interviewing—both had beards that covered their chests and they were dressed casually without ties.
I didn't get the job, but I imagined them chatting about me after the interview, saying, "Paul was quite well-qualified, but we could never employ such a conventionally dressed man who barely has a beard!"