I was 16 years old and i'm confused, answer my question with your suggestion please...

i've been studied in china for 6 month with visa L then changed into X2 and "are there age restrictions to the R type entry permit?" (*From the precious question by Mr.Martin USA) i mean when i want to change into entry permit R the CVASC said i havent 18 years old? or maybe the problem is i have not JW201 / JW202 ? if age restrictions is 18 years old - 65 years old, so what type should i use for study in china for 3 years and i still need to go back to indonesia every 6 month, should i apply X2 every 6 months?