If you are 15 years old and you have sex, do you stop growing up?

No. There are a whole host of reasons why sex is not a good idea at age 15, most of which are psychological and not medical, but "stop growing up" is not one of them, .
No. Life's difficult enough without people telling untruths to frighten young people. Decisions about sexuality are personal. Should you choose to start, i hope you'll love wisely and safely, and that your introduction to this aspect of adult life is a wonderful experience for you.