Question about braces?

Hey guys, so I have braces and lately my orthodontist has been telling me that i have to wait untill my back molars are fully grown in to put braces around them also. My problem is, who knows how long its going to take for them to grow in? For a few months now all they do at my appointments is change my colors. Its agrivating because at this rate i dont feel like im going to get them off the summer of 8th grade like im supposed to. The worst part is my dentist tells me that usually orthodontists dont put braces around these certin molars. Does anyone know anything about this and if i will be able to get them off in time? Sorry that this is kinda long but I've been wondering for a really long time. Thanks(: