I would say yes, a 13 year old boy can manage a Dart16.

Ofcourse assuming that he's not sailing on the sea and in moderate winds.
Give him a good fixed lifevest with a collar (not the one that blowup). And a cell telephone in a watertightbag.

And practice with him righting the cat until he can dream it.

You've probably heard of the Dutch girl which sailed solo around the world in here 16th year. She sailed already alone across the North Sea to England at 14!