Many years ago, I remembering reading a story about 4 young people who were older than they looked, but slow to mature physically because they were a new variation of humanity. I seem to recall one or more of them had a musical group (on the moon?), but each of them had some area of expertise in which they made enough money to finance their plans to leave the solar system. They may not all have been musicians. At the end of the story, they are thinking of heading out to the stars. I believe it was one of the young adult novels similar to Heinlein's but I am pretty sure it was not him. I know this is vague. They are two young men and two young women. Seems to me one of the women was a financial genius. Not much to go on - sorry.