There are a handful of reasons:
1. When I was first coming out when I was 20 or 21 years old, I chased all the daddies. At the time, guys my age did nothing for me. Anyone 30, 40, 50... they were totally hot to me. I like the experience they had, the calm and comfortable environment they provided, and the way they interacted with me. Now there were some exceptions with a few creepy older guys, but for the most part my judgement was sound and I found some really good older men who gave me some awesome times.
2. Right around turning 35, I turned into the daddy. My hair went almost completely grey from the ages of 32 to 35, and I kept it short military style which they all love. I am naturally bigger (220# to 260#) and pretty masculine. Then have a few wrinkles in my face which many say is pretty handsome. Pics in my profile, so you be the judge. My point is that a light switch flipped and I found myself attracted to the younger college 'bros' who were the same guy I was, chasing after me.
3. Fast forward another decade, and we're here today at about 45 years old. I have a good amount of life experience under my belt, I am very successful in business, and I am the textbook college football coach in real life. Hell even my friends of the same age call me daddy. Some gay guys hate getting older, but I am kind of loving it at this point. Can't complain at all.
As for what turns me on about younger guys, there's obviously that "new car smell" thing, right? I mean, finding a hot 21 or 22 year old lacrosse or soccer player with daddy issues and a stiff cock that can go all night is absolutely fucking amazing. Smooth skin. Solid body. Sex drive off the charts. Check. Check. Check.
But I also love being an older, stable, good influence in their lives. It's almost a responsibility to pay it forward and 'protect' the younger generation from those older creepy guys that my own parents warned me about. In fact, today I am actually that guy my parents warned me about. I remember the first time I fucked a dude born in the 90's. That was a little mindfuck to me. Now today, there is going to be a point where I put my dick into someone born after 9/11 and that's going to be even more crazy. LOL.
The whole dad/son power dynamic works for me. I'm also naturally a top and It just turns my crank and it turns their crank. It's a win-win in my book. I would never take advantage of someone based on their inexperience or age. I will only ever fuck someone under 30 who is emotionally mature and after knowing them for a little bit. There's also a handful of younger jock type guys that want to top me and I am more open to it now than ever. I enjoy making them feel good and most importantly, feel comfortable.
So yea, the age difference can be hot and it works both ways.