There is no way to control this. Any parent can hold back their child if they want to. All they have to say is that they don't feel that their child is prepared for the next grade. This has been going on in football and basketball for decades.
TRN is done by a player's year in school, regardless of how old they are. Just like scout.com and rivals.com do with football and basketball. They don't list a player's birthdate and they are owned by major corporations.
Once a player turns 19 then they are not going to be playing any tournaments that will be on the TRN system. If a player turns 19 at any point in a calendar year then they cannot play Junior ITF tournaments in that calendar year.
The USTA stopped putting DOB up years ago because of legal issues. At the BG16&18 Spring Supers they put the players DOB on their player cards that are put on the fences by the courts. One parent demanded that they take their daughter's month and day off the card.
When an 18 year old freshman has to go up against a 23 year old senior in a regular match in college nobody cares about age. That is the position you are in and you have to fight your way out of it.