If i start using steroids when i am 19, would it make it harder to build muscle later without the juice?
There are probably no studies on this, but I'd say yes, as it might impair the full natural development of your hormonal system, and that means you would never attain the natural peak levels you would have otherwise.
I don't know. Arnold started using using he was about 16 from what I understand.
How is this relevant, as Arnold never went off? OP specifically asked whether early use would matter when someone stops using.