Learnt when 11, SSA Instructor by 12. Am 38 now and still dive actively. Am of average height and intelligence so it didn't seem to effect me. Had 500 dives by 18. Have broken a few bones on my motorbike, but they all healed normally and X-rays all look normal. My legs still seem to work fine despite what everyones told me.
So in short dont worry
The main drawbacks of learning to dive that young were
1) the 4cuft cylinder was bigger than me and I had to throw it in and put it on in the water
2) My twin hose DV could wrap about 6 times round my small head
3) My brain is now convinced it can breath underwater which is fun when I go normal swimming
Good bits
1) Wetsuit was a lot cheaper to make as I needed less material
2) Telling adults what to do
3) A 40cuft lasted for ages as my lungs were so small
4) I got to wear a BIG knife!!
BUT I'm also fat and ugly, So as we now have 2 documented young divers whom this is true for have we spotted the side effects
So in short dont worry
The main drawbacks of learning to dive that young were
1) the 4cuft cylinder was bigger than me and I had to throw it in and put it on in the water
2) My twin hose DV could wrap about 6 times round my small head
3) My brain is now convinced it can breath underwater which is fun when I go normal swimming
Good bits
1) Wetsuit was a lot cheaper to make as I needed less material
2) Telling adults what to do
3) A 40cuft lasted for ages as my lungs were so small
4) I got to wear a BIG knife!!
BUT I'm also fat and ugly, So as we now have 2 documented young divers whom this is true for have we spotted the side effects