It is probably helpful to clarify and expand on this question. Are you asking about legal requirements, specialty certifications,the physical and anatomical differences to be taken into consideration when working with youth? Do you mean youth, or young adults or children? Are you thinking about developmental issues?
This is a big question you’re asking. Young people aren’t just little adults. There are neural and physical development differences, and depending on the age group, their needs can be very different.
The gym where I work has minimum age requirements for youth attending fitness classes, so I don’t typically work with youth under 12. And what I teach is within my scope of practice as a group-ex instructor and a personal trainer.
No matter what population you’re working with, it’s always helpful to consider your scope of practice. What are you certified to do? What are you qualifed to do? What are you insured to do?