I'm a Fencer! I fence in Colorado. I started at a club called Fort Collins Fencers (FCF) and now I'm part of the Univerity of Colorado's fencing club. Other places in Colorado to fence are the University of Northern Colorado's club, Denver Fencing Center, Mile High Fencing Club, Northern Colorado Fencers, Pierre Le Chat Fencing Club/CFA, and Pikes Peak Fencing League.
To find out information on fencing clubs in your area, then you need to go to the United States Fencing Association's website:
http://www.usfencing.org/
On the left side, there is a list of links. Click on the "New to Fencing?" link and a red list will open up. Click on the "Where can I fence" link (fourth down in the red list). Click on your state on the map it opens up, and it will show you a list of the clubs in your state.
You can never be too young or too old to start fencing. If someone told you that you can't fence as a kid at one club, then they merely don't have the equipment in kid sizes at that club, and I'd recommend finding another club in the area that does. Fencing is a fun sport, but it can cost! I've been fourtunate enough to find great deals at the clubs I fence at, but that's rare. Most clubs are very expensive to join and fence at, especially if you have private lessons and a personal coach. Personally, I've never had a private coach. I fence for the fun of it.
Fencing is low key in the United States, at least in the Western US. I hear it's very popular along the east coast.
I hope that helps.