16 is definitely okay. I didn't switch over to MMA until I was 16. (I'm 17 now)
I started with high school wrestling, American Kenpo, boxing, and a martial art called Krav Maga. Those all kind of gave me a base to work from. I did these from late 13 to when I was 16.
If you're athletic, it'll definitely help. Before you look for a BJJ school or anything, see if there's an MMA gym first. People these days make the mistake of thinking that MMA is still about training in a couple styles and trying to mix them. It's far beyond that at this point. MMA is its own beast; you need to train MMA to be good at MMA.
The other thing is, don't get too caught up in cross training. There are lots of good articles on this site about training for MMA but there is NO, I repeat, NOOOOOOO substitute for technique training, drilling, and sparring. Strength helps in MMA, but when you start, you are going to be baffled that the little guys can tool on you in the gym.
To the guy who talked about MMA fighters making "peanuts:"
There is an epidemic in this country of not giving a shit about anything but money. I really blame our obsession for money and possessions to be the root of the majority of problems in this country, including our huge lack of knowledge for other cultures.
I find it sad that you did that well in BJJ and decided to call it quits. True, the local fighters don't make much at all, but they don't fight full time. They have other occupations.
Meanwhile, at the top of the chain, Pride and UFC, those guys are making plenty. They may not be overpaid, spoiled, multi-million dollar baseball players, but they definitely make enough to own a nice house. Plus, you forget about how the sport is growing. $$ is made in endorsements, my friend. Have you seen Arlovski outside of the Octagon? The amount of publicity he got, and the money he got for endorsing supplements, the guy was buying sports cars, donating to charities, having a custom medallion made, and getting EXTREMELY expensive massage therapy. If you consider that "peanuts," I really think you need to take a look at how much shit you actually need.