When they're a bit "lived in" and have been around the block a few times. I find nothing more sexy than a few lines on a face and gray/ salt&pepper hair. Just can't understand why guys would want to go under the knife and please please please just bin those "men's hair dye" products (you know which ones I mean :wink
Also, age is just a state of mind. After I had my 40th, my life changed quite dramatically for the better - I left my job, retrained in something I've always wanted to do and after I finished (with special award et al) my training place offered me a paid job. Best move ever and no regrets.
Having said that, I do believe that each individual has their own "prime time". Some look just perfect as kids, others are stunning as teenagers and others (myself included) have to wait until later in life. I remember some of my class mates were very attractive as teenagers but seeing recent reunion pics they just have not aged well at all (balding, paunch - not in an attractive way, though).