Need age fast? Then paint it! There are lots of textured paints that can give an aged appearance almost immediately, depending on the material you want to age.
Rust isn't just a color. It's texture too, and some paints that are available actually contain some kind of material in the paint itself to give a texture to your piece.
I've used this on a plastic bird cage to make it look aged and rusty. The one problem I found was that some products flake off in the weather. Double check that you have one rated for exterior use, then also prepare the surface that you want to paint.
Sand the piece to give it some 'tooth' and then wipe all the dust off with a clean rag.
I know it's tempting to just start spraying, but as you can see from my bird cage above, the paint won't stick evenly and it just looks terrible, so take that extra step.
For metal, such as a filing cabinet or other piece, paint with Tremclad rust paint for the first coat, then spray a quick layer of textured paint over top. The Tremclad rust paint is a matte finish paint meant to be used as a base coat.