Nice to see the new section.
As to what is deemed collectable, just about anything! God knows why someone would want a collection of AA S4*0s but if that's what floats your boat then so be it!
I still don't consider myself a collector although I am amassing a few interesting (to me anyway!) bits and bobs. Generally I go for British stuff in .177 only (I'd take .25s if I ever came across them in good nick), which often makes things more difficult for me. Age is no particular descriminator for me be it a year old or fifty I'm not fussed, I'm more keen on the condition and them being relatively unusual due to the calibre and/or condition. I'm not keen on non-standard stuff or non-original manufacturer accessories. The newest rifle I have got just for the sake of it, i.e. serves no real function as a hunter or competition rifle, is a Birmingham Webley Xocet and the oldest a Webley MKIII. There's a couple of BSAs,Superstar and RB2 Airsporter an Air Arms Pro-Elite and a Theoben Sirocco Grand Prix in between. I'm no real fan of pistols but I do have a boxed BSA 240 Magnum lurking too. I'm also a sucker for putting period optics on them rather than the very latest. I particularly like some of the older Nikko scopes and the Japanese Tascos can be stunning.
I've just picked up a Nikko 3-9x40 MRE mainly for the novelty value. It's supposed to have pretty good optics and choosing the different reticles will be interesting although I doubt if it will be genuinely useful. God knows what I'm going to put it on!