Student or Learner
"A singing teacher took his students to sing in a charity function. Those kids were very young (like 6, 7 years old). People there asked the teacher that why he brought little kids for singing he should have brought juveniles (like 17, 18 years old)."
"Juvenile" doesn't really have a specific definition so the person might not know what was meant. Your original suggested that 17- or 18-year-olds would have been better so the person organising the function could say "Please don't bring any under-17s" or "Please make sure the choir is made up only of older teenagers". Instead of mentioning the performers, they could say "Please note that the environment is unsuitable for under-16s".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.