I found another post on here from Bryan that state full maturity is reached at 10-11 months. At 6 months they are mature enough to breed, but I was under the impression that there could be some filling out that is done until 10 months.
I have had babies so rarely in my history with hedgehogs so my actual experience with it is very limited. I typically get hedgehogs when they are over a year already. The first babies I don't rightly remember when they stopped growing (was 14 years ago), but with Riley & Rose I know Riley continued to fill out until he was nearly 10 months. He was such a slender boy at 6 months, but filled out to be quite a handsome boy.