Which is better?

Is it better one on one training with a master martial artist or with kids or teens near around your age to train with, when a teacher/ martial artist is training everyone at once. Why is it better?

I'm not sure I understand your question. Do you mean in a class environment vs. one on one training?
In that case I'd say in a class environment.You can train with a variety of people instead of just one who can already beat you.

whateva u do, try to pick a class with kids your age, never, NEVER go to a class with about 15 under 10yr olds, and bout 5 other people your age, its better to have a mixed class with kids ur around ur age, allll different shapes and sizes

It's actually better to train with a mix of both. Having the experienced instructor training you helps you refine your technique, but having several people of varying size to train with helps you to adjust how you perform your technique in different situations.