Every state is different. In my state first offenders can get charges similar to nolo - they're charged but if they comply with probation then it completely comes off their records. most other charges are sealed automatically for minors except for drug charges - however the county gives them the ability to have those taken off their record as well if they stay out of trouble. If they get out of the control of the county level then they are transferred to the State then those are harder to have sealed because at that point they feel that the kid is pretty much going to be a repeat offender. At 17 they treat minors as adults and if they are serving time in youth detention when they turn 17, then depending upon their crimes and their record they may send them to adult detention.
In my state, youth do have a public defender but it is very different then if you get your own attorney - its more of a guardian ad litem. I'm in the legal field and was a bit taken back when i sat in my son's first hearing - I kept waiting for the attorney to object to things and it never happened. So I researched their role and found that the public defender is in a different position then a regular attorney - their goal is not to just get the kid off but to help get what is in the best interest of the child. So if the child needs to be in custody so that they can get substance abuse help then that is what the public defender is going to go for. In my AS case this was the best thing for him so I never brought in my own attorney.
I dont know if my son's charges will be purged in the future but its his problem not mine. He doesnt care why should I? He did the crime - he suffers the consequences - it has nothing to do with me. Trying to defend him and get his record cleared only tells him that I dont think what he did was bad and he'll expect that forever. But if you want to know you can talk to the public defender and they will explain your states laws to you.