When I was younger, in my 20's, I was attracted to older men 10-15 years older. When I reached 35, older men no longer seemed attractive, they were just old.
To date a man 15 years older at 20 is great, the man is still young, but when you are 35 plus and you think to the future, you consider that when you're heading towards 50, you may have a grandfather, to wheel around in a chair. Shallow maybe, but it would have an impact on the relationship.
Now I'm nearing 40, then the ideal age for me would be the same age or maybe a couple of years older or younger. My partner is 8 months younger than me, which suits me fine.