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.