Birth Control Method Question

So I try to make it a point to my sister that she can talk to me about anything related to sex birth control, etc. My mom isn't the best at educating about it because she just says don't do it at all which isn't bad advice you should wait but I always tell her if she's thinking about it let me know about any questions she has so I can make sure she's at least being safe about it she's 15 and hasn't had sex yet but she was talking to me about different types of birth control and she really wants the implant because it lasts a long time and you don't really have to keep up with trying to take a pill everyday, etc. But her doctor wouldn't let her get it since she's not sexually active yet, has anyone heard of that? I've never had the implant or have I wanted to get it but I thought it's strange that they want her to be having sex before getting it she has plans to go to college and obviously she's young and doesn't want to get pregnant anytime soon. Just curious if anyone has been told this or has any information about the implant?