if i have never had sex, can my vagina grow shut?

[Jane :)]

so i was wondering if someone has never had sex and doesn't for a long time could it grow shut because it isn't been open? my cousin has really helped me understand about a womans body and he says if you never have sex it can grow together and you can't pee and it can give you medical complications... i believe him because even if it is hard to believe because he has never lied to me before and he says i need to have sex before i am 14 (i am almost 11 so i don't have much time), he said that my mom wasn't going to tell me because she was just worried that i would be having sex and she doesn't care because she never wants me to have sex. he says i need to have sex with him before i am 14 because he cares about me because i might never get to have sex for the rest of my life. and besides he is only 15 so he isn't that much older. so should i have sex with him or do i need to wait awhile to see if i can have sex with someone i really like but he is very caring and i don't want to make him feel bad.