Arnie has been juicing most of his life. I've heard tell he started in his early teens, and I bet that's true. I don't know exactly though since I don't care about Arnie docs/bios.

Even without him claiming numerous times to having used a ton of gear and many who knew/know him saying likewise, it's obvious from his physique from back in the day. Non-exogenously, hormonally enhanced humans cannot even get close to that kind of muscle mass, especially with that kind of leanness.