I'm surprised you're having success with the splitty at such a young age. (But, happy to hear it's working for you, of course.)
Kyle Farnsworth (former Cub, now Yankee) showed me how to throw his splitty in Spring Training a few years ago while we were both with the Cubs. Farnsey is a giant: 6-foot-plus, strong as an ox. He essentially said the key to a good splitty is having big hands, long fingers since the pitch is choked so deep in the hand.
As a result, it's been my experience that most young pitchers (by young, I mean pre-college) don't have big enough hands to throw it. So it doesn't come up.
Like any pitch, the magical age to throw it depends on your physical maturity, how long you've been pitching, training, and conditioning. But if the pitch is working for you. You have no problems, I don't see why you couldn't keep throwing it at 16.
A lot of big-league pitchers are in the big leagues because of their split-finger fastball (though most learned it later in their development as a pitcher).