Your pending I-485 is still valid unless it is denied or approved by error. After your parent files I-290B Motion to reopen his/her I-485, your I-485 will continue to be valid. Because you have pending I-485, you cannot get the F-1 visa or status due to immigrant intent. But you can just continue your study even if you turn 21 years old because you have the pending I-485 (you do not need to do anything). Don't leave US before you get the green card formally unless you get the advance parole.