Whenever they want to and ONLY if THEY want to. Forcing a child to do something because it’s what you want won’t help them in any way, and it won’t cause them to feel motivated. If a 10 year old wants to lift weights, let them, as long as they are doing it safely! And if a 15 year old doesn’t want to live weights, don’t force it upon them. Life long healthy habits can only be maintained if the person creating the habits actually wants to do them!
Definitely can start with small weights during middle school. No big weights should be used until at least 10th grade maybe? Then again everyone is different
I don't know correctly about other, but i started lifting from my 16s. And that's great for me, keep working out and remember age is not a limit when it comes to exercise.
I think when they get use to using their full bodies in gym class and learn about minor exercises that move a specific muscle would be a great time for them to lift small weights. I didn’t lift weights until 6th grade. My gym class taught us about pull ups and how to work the body in different areas. We started out with 3-5 lbs just to see what it was like. As I got older, I loved what weights did for me and would incorporate them in my workouts.