Almost all my FB friends are people who play the same FB games that I do. I have a couple who I went to high school with, but they invited me. I have one friend from 10 years ago that I invited, we haven't spoken, but he accepted. My last friend is a family member.
I know of many other people I'd like to add, but I too, am uncertain how they might react to my invitation.
I would suspect that the appropriate way to add a stranger, would be to send them a message first explaining why you want to friend them (for example what specific interests or things they are posted that makes you interested in them). I'm not sure you can browse people by age groups, since more of the search options focus on schools and work. You could always just pick a random school and give it a graduation year in your age range. That's about the only way I think you could do that.
And if anyone wants to encourage me to invite people I've known from the past, please don't hesitate.
_________________
On the Spectrum since 2003.