DD-214 in hand and I'm gonna party like it's 1998
"Old man brain" is a real thing- the failure rate for 55+ and especially 60+ is shockingly bad. The older you get, if it's your first exposure to big wing flying and flying the 121 way, then the learning curve is really steep. Working your ass off and having positive attitude only goes so far and at some point it's not enough. Time is money!As more and more RTP pilots enter 121 training there seems to be mixed opinions on their performance. Some do fine and others don’t. Extra training events are sometimes required which airlines don’t like.
Age takes a pretty big bite in your aptitude to learn new things, to learn a lot of new things, and most of all to learn them quickly. It would be like someone who's only flown heavies for a decades trying to learn helicopter flying late in the game (not that the market forces will ever drive that). All the things that are typically difficult for twenty-something brains to learn would be a little harder for forty-something brains to learn and that much harder for the fifty and up crowd.
Normal maneuvers and normal flying (basic IFR procedures and whatnot) aren't difficult in any kind of flying, airplane or helicopter or anything else; it's the abnormal procedures and emergencies that are hard to learn- on top of getting your flows down and managing the automation, both of use up a lot of study time and work (brute force memorization). It doesn't mean it's impossible and it's far from it, but it's like you don't know what you don't know.
I do think the regional training departments are gradually adapting to this aspect of their recruiting pool and I guarantee that they're struggling with it, even if they haven't yet decided what to do about it.