I too find this BIG TIME when making friends. That the best friends I make are significantly older, or significantly younger. They can literally be all ages apart from under about 18 (for obvious reasons) and usually apart from around my own age (for obvious reasons to most Aspies). So basically they're usually any age apart from mine.
Not a problem as an adult, but it's sure seen as a big problem prior to that. If you had a gap in friendship of more than about a year, then it WAS seen as abnormal to say the least on the part of the person who was older, particularly if they were a guy. Especially with the problems I had relating to people my own age, I must confess I did fall foul of this a lot. And there was no question of me still being in touch or socialising with people I went to school with who were my own age - or even those I went to university with, as the problems I had relating to my own age (and the problems people my own age had relating to me) only continued.
So I'm glad I'm not in any sort of academic setting now, school or university, where you are necessarily stuck with your own age, and I'm sure that as long as I live my best friends will be very different ages. I'll be in my 60's and my best friends will be people in their 20's probably, that's just how it is with me, and it won't be seen as inappropriate since at least they're adults.
The age difference in friendships thing must have implications for who Aspies can attract as partners. If say I was 35 and have my closest friends as people who are around 20, and people who are around 55, surely this would suggest that if I had a chance of a partner they would also be someone similarly outside my own peer age group?
It doesn't just stop at age of course. What about ethnicity and national background? That's a similar scenario, where I have become friends with people who originated from other countries or at least have backgrounds from other countries, more than other people have done.