It was 15. I read the new book about him. It said that he was brought into steroids at age 15, but then it was average...like creatine today. Note: He did admit to it...he said it helped him 10%...LOL!!! like 200%.
15, no doubt. Look at pics of him around 16-17. Even if you've been training since your mom popped you out of her vag, there is still no way you can get that big without anabolics. Doubly, in his biography he said he started to workout around 14 (pull-ups, running, push-ups) so in two-three years he gets f'ing huge. I don't think even with arnold's genetics he could have put on the size like he did without AAS.