Im 22 and my version of old is quite dynamic. There are guys my age who are overweight, don't take care of themselves and look really old. Then there are 28 year olds who look really young. It all really depends.
I think most guys 28+ have usually stopped taking care of themselves for some reason. Mainly their body fat is way higher. And higher body fat on anyone makes them look average and ugly a lot of the time. It's not enough to just fit into a size medium. After age 28 ish the body starts retaining more fat, metabolism slows down and it's blindingly obvious which guys make the effort to stay ahead of that and those that don't. And most guy's don't, and thus don't appeal to me. And not just body fat, skincare, hairstyles, everything. Younger guys in 20s simply put a lot more effort into these things usually, and that is a big factor in why they are considered more attractive. IMO.
tl;dr I care about those things rather than age. I don't think I'd ever go out with a 40 year old. But even though I consider age 32-35 too old also I can definitely make exceptions if they put the effort in and are still slim enough to have abs.