64-78, but if they're in their late 70s, I don't want someone "old", mentally speaking. You're not "old" just because of chronology: you're old because you're no longer curious about things, and you still cling to ideas you believed at 30, and you aren't alive to new ideas and new ways of looking at life, as Einstein once said (Actually, he thought anyone who wasn't curious about the mysteries of life were - as he put it - 'already dead.') He didn't mince words about that observation! And that could be someone who's 28 (although that would indicate an overly stoic disposition - or depression).
Young guys look great and have lots of energy, and they're nice to look at, but when I listen to them at the gym (which is right near the high school), it feels to me that they are, as Bette Midler put it, in The First Wives Club, when asked by Goldie Hawn about Bette's husband's new girlfriend and how old she was: "Twelvvvveeeee." (it was Sarah Jessica Parker, and she was actually 24, but you get the idea).
Besides which, I prefer someone who is able to converse with me and look at me more often than the look at their smartphone for the new text message that just came in (as though they can't wait to read it later, which says A LOT) and can actually discuss (and remember) history. I also like the wisdom and insight factor, but I can tell you, there are PLENTY of guys in my age group who are complete adolescents, emotionally speaking, although it's not hard to pick out those whose emotional development is limited. They don't understand the deeper emotions, and have a surprising number of fears which, when you are closer to the the Close Of Life than the Beginning of Life, you'd think they'd have worked those things out by now.
I think, though, it's an American thing. I've met French guys who, at 35, are more mature than American men at 55. That said, unless I'm living in France (which I'm not), I'll stick with guys around my age and older. They actually appreciate life more because they've been through the ups and downs - and they also realize they're not IMMORTAL. When you realize that you won't live forever, you look at life differently. And far more appreciatively. Of course, younger people who got cancer at 24 are likely to be much "older" (aka: mature) than their peers who never had a serious illness a day in their life.