Is this for an indoor or outdoor item? Is it rust or just a darkening you're trying to achieve? 

This isn't something I do, but I can pass along a couple links until someone with more practical experience comes along. There is quick method described here: http://www.artmetal.com/images/sculpture_gallery/josh/2007/07/rusted_sculptures_update_0. A not-as-simple method described here: http://www.artmetal.com/blog/eligius1427/2009/01/windvane. You should be able to find more using the search box.

The Jax Chemical Company also makes a range of products for producing patinas. You can check out their web site for more info.

http://www.sculptnouveau.com

Sculpt Nouveau is a great place for metal patinas. They are also very helpful over the phone,
Mele

put iron in forge and bring it to near welding heat, turn off air and leave it to cool down this adds 100 years over night

It all depends on what you mean by aged? There are so many different things you can do... Try to be specific as to what you're after and I totally can help you. I've been a patina artist for over 15 years.
Matt Hart