How old do i have to be to get a tongue ring ?

so i am 14 and i want to get a tongue ring at 15. is there any way i can convince my dad to let me get it? i already have mi ears 3times my nose stud. my Monroe and my belly ring. so my dad said that he wont let me get any more till am 18 but i want this oe so bad. hlp LZ. and this is not for sexual use i just like it i ike how it looks and i am 14 and i love getting piercings but this is the last one i want. so plz help me convinse him to let me thankyou. one more ting am sorry for the spelling i just moved to usa.