I think it's perfectly fine, as long as they are very experienced, have great, GREAT patience, don't rush past essential stages and can see things from the colt's perspective. If they are the kind of person who tells a horse off for bucking instead of looking for the reasons as to WHY it is bucking, then they should not be saddle breaking a colt. Not until they have a much better understanding of horse behaviours and instincts.
It's an interesting question actually, especially since most would disagree with a teenager backing and breaking in a horse. That said, I'm currently breaking my mare to ride and school, and she's doing incredibly well, yet I'm 15.
So I guess age does not matter as long as you know what you're doing and have put in the time, research and have patience to slowly back and break in a young horse.
I'd say if they have the confidence and they are experienced enough then go for it
depends on how experienced they are.