The provision you cited is 948.60 (3)(c). Keep reading and look at 948.60 (3)(c). That section says:
" This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593. "
941:28 is a ban on short barreled rifles and shotguns, so presumably that does not apply, and 29.593 only relates to the need for a hunting license when hunting. 29.304 when you actually read it has regulations for 16 and under.
So a 17 year old is only subject to 948.60 if they happen to be carrying a short barreled rifle or shotgun.
Poorly drafted laws, but clear that they don't apply to Rittenhouse.