I don't fully subscribe to that theory. It depends on each particular airline and what they are looking for at any one time. An airline full of twenty somethings is in for a rough ride at some point. Assume they all stay there, they would all retire together.
Younger pilots are more likely to move jobs so there is an argument which says that younger pilots may be more of a risk. Spend thousands getting them type rated only for them to leave after a year.
Older pilots may take longer to train = more expense, so maybe they cancel each other out on a risk basis ?
As for medical, well anyone who is fit will have a Class 1 and that is all that matters. Ill health = no class 1 = no job, whatever your age. Is someone at say 34 at a far greater risk of ill health than someone who is 24 ? anyone got figures ?
It's horses for courses. There are jobs for a wide age range. The hard part is finding them.