A few problems:
1. Cadets offers a chance to be military adjacent, without being military for kids whose parents wouldn't allow them to sign-up for the reserves. You'd likely sour quite a few if at 16, two years before being able to make their own choices, you cut them loose and said "too bad, sign-up when you're old enough".
2. Cadets corps exist in places without a convenient reserve unit. You'd sour those kids against the military by cutting them loose years before they can join.
3. Most of the programs that draw people into the Cadets programme are only available when you're 16+. Things like international exchanges, power pilot, etc...