Beset by Owls
Eras, Dates, and Schoolmates
I'm terrible with dates, and when I tried to figure this out I only got even more confused. So, let's say someone (Benjy Fenwick) worked alongside the order in the 1970's. That's what the Lexicon said. So who would he have gone to school with? Not the Mauraders and Lily, right? Would any member of the Black family have been there, because it's important to my story that they are.
Well, I'm not too sure about the Order, but I know that maybe Bellatrix, Lucius, and Narcissa might have been in school with him.
Snape Hates Me
I agree. I don't think we have any description of Benji that would determine his age, do we? I only remember that he was in the photo and found in bits. I think you can make up his age which gives you free reign.
I think it would be safe to assume he might be at least a little older than MWPP since they were rather young. According to the Lexicon, Lucius Malfoy was born in 1954, Narcissa in 1955 (according to the Black Family Tree that JKR released a couple of years ago) and now we know James and Lily were both born in 1960, so Lucius and Narcissa would have been at Hogwarts Before the Marauders for a little while. Lucius in 6th year, Narcissa in 5th when they started if my math is correct. If the Black Family Tree is accurate (which it does not appear to be completely, as there are some contradictions with it), Bellatrix was born in 1951 and would have definitely already left school before the Marauders, and Andromeda was between Narcissa and Bella, so Benji could have easily been there with at least one of them.
I agree with everything MoonyManiac said (and every reason).
I'd also like to elaborate that it's entirely possible that Benjy could have been a contemporary of Mad Eye Moody's. Or even any of Cygnus, Wallburga, Lucretia or Orion Black. You could even go a further generation "up", as there isn't much difference between them.