Talking to a lot of people, I do believe older horses are more...disciplined. 20 years ago, they weren't treated like big puppies. They had firmer, if somewhat mean training. They knew the boundaries though. There also weren't as many short cuts and people watching their favorite TV trainer and trying to train that way. These oldies tend to have a better foundation.
But they also tend to get stuck with the newbs and the bad riders, they learn bad habits, they get away with to much.
Ty when I got him, was barnsour as hell. A previous owner had let him get away with it. I was 8, so he got a few rides by a neighor's horsey daughter. The second he realized that he wasn't getting away with it, he quit it. He knew better, but was seeing if he could keep getting away with it. Finding he couldn't he stopped.
So yes, some oldies seem like nuts, but often I find they turn into amazing creatures with a good rider.
Bailey, a 17 year old QH I get paid to care for, is the same. He has a fantastic foundation, no gaps that I can see, but again, a young rider let him get away with ignoring leg. 3 rides with me, and he just needs a cluck to go, and turns beautifully off of leg.
There are of course plenty of exceptions but I've worked with tons of older horses, and I find that to be the case 80% of the time.