I don't think it's bad to do it in moderation as long as you get a pro - ie. pitching coach - to approve that he is throwing it correctly. I got the green light for my son to throw his from multiple resources before I let him throw it.
I also agree that a good fastball and CU should be developed either before or while you throw the breaking ball. You will need them as you get older. My son, at 10, had good command of his FB and threw a wicked CU. He would corkscrew batters into the ground with it. That FB/CU combo worked great for him until he got to be about 12.
The problem was that he threw harder than most at his age. Not many could catch up to his FB. When guys did connect, hit balls were to right field. Throwing a CU would bring the speed down to their bat speed. He did develop a breaking ball that was pretty effective and would use it as his K pitch. CU was still used on occasion with some of the bigger, stronger hitters he faced, but overall for 12, 13 & 14 it was tough to use the CU alot. We did continue to work on the CU in lessons and bull pen sessions.
This year he is a freshman in HS. He pitched as a starter for his varsity team. His CU has been VERY valuable. He has thrown the CU way more than the breaking ball. He pitched against the (at the time) #1 team in the largest division in the state (they wound up the season at #3). He lost to them 1-0, but only threw 3 breaking balls the whole game. It was all about the FB/CU combination. At the HS varsity level, hitters can adjust to any speed fastball, so you really have to keep the batters out of their rythm with the CU. BTW, he is throwing mid 80's FB, low 70's CU.
My point is that I don't think there is a problem with throwing a breaking ball at the earlier ages as long as you make sure you are throwing it in a way that is mechanically sound and you continue to work on the FB and CU. They are both very valuable pitches.