I have bought a number of used Brooks saddles, mostly B-17 narrow or similar (including a couple of B-15s). Most of them appeared newer but a couple had 60s or 70s dates. The ones I put significant miles on were all toward the newer-looking end of the spectrum. By significant miles I mean in the 500-1000 range, not the 10,000 range some folks here would consider significant.

Overall they have all held up well - in fact, I can't think of one that has started to deform noticeably or tear at the rivets or anything like that. I've also bought some bikes that had Brooks saddles that were severely deformed, but since that hasn't happened with any of mine (yet) I can't say what the risk factors are. I mostly ride these in fair weather, so I assume a lot of water would be one source of trouble.

Anyway, my experience is that a good-looking leather saddle is a pretty reasonable risk as old bike parts go.