Kids and communication

Ok, so in chapter 5.2 they talk about inner speech..well this blog has nothing to do with that..but it did strike up a random thought... you know how some kids have thier 'own' language they share with siblings and peers..is that for real? I mean are kids capable of coming up with their own language and actually understand it? or are they just making weird obnoxious noises and saying its theirs? like twins i guess....they are so close they can feel each others emotions suposedly, and i was watching law and order:svu where a set of twins communicated in there own language.(law and order:svu probably isnt the best source for info but its a great show!) do kids just want to believe they understand their languages and go along with it, and make up what the other kid could be saying and goign off that? or is there actually some thing there? I mean my mom said that me and my brother had our own language and that is why i didnt talk till i was much older, because he did it for me, but, i think she is just crazy. so yeah, that is my question, can kids make up their own language while their imagination is still soo very active and growing, and then lose it when they hit a certian age?