We do not know the answer to this. The book never actually tells us the age at which girls get their hair ribbons. We do know the age at which they no longer have to wear ribbons in their hair. We find that out in Chapter 2. Jonas's father tells him then that he remembers when his sister became a Nine and was allowed to take the ribbons out of her hair. However, we are not told when they start wearing them. So we have to assume that it is fairly early, at a point when their hair has gotten long enough to braid.