Junior Lifting Risks

I turn 16 the 5th of march.
I had a very long conversation with my gym trainer today about how he feels i shouldn`t lift the way i do.

He thinks my form and technique are all good, but he doesn`t like the way i train to failure after 8-10 reps, with progression as goal.
He says im too young and when your still growing this will cause damage to tendons and joints, giving me complaints in the long run.

I have known about these stories for a long time, but i thought they were only true when you use anabolic steroids too soon or have bad form or do something else wrong. I still believe the latter is true.
i think you will agree but i want to convince him. its not like hes going to make me stop lifting in the gym, but hes made it pretty clear he doesn`t like me training in this way and he seemed very serious.

So i was thinking, Arnold Schwarzenegger was really really big and strong already when he was around my age, now hes closing in on 60 years age. He has never had any problems has he? Same goes for for instance Shawn Ray and many other bodybuilders.

Another employee in my gym was deadlifting 420 lbs when he was my age.
He never had any problems either.

I want the technical information and details about any risks in natural bodybuilding in adolescence, with correct form.

i dont think there are many, but this trainer is convinced there are.
Please tell me what you think, and if you can, prove your points and/or give me some referances.

I want to go on, my lifts arent that big at all and i want to continue but how do i convince him? Its not like hes just gonna take my word on it when i say "thats not true" I`m 15 years old and kind of new to lifting, hes 28 and has an education in it.