Actually, if you wanted to be "authentic", the Age of Steam does have definite dates associated with it. Example: Though steam inventions started waaay back in Alexandria 2,000 years ago, in more SP terms the first US steam engine to carry passengers didn't appear until around 1830. I think what most SP'ers are looking at for inspiration is the latter part of the 19th Century.
For the steam part, anyway - the tech stuff is definitely all over the board
And yes, there are SP'ers who are busily modifying their Nerf guns to be steam-powered photon emitters - others are making the guns from scratch with wood, brass and iron.
My favorite is still all the tech stuff they mod - the computers, etc. It's just a great visual.