? They just grow to full maturity faster than humans. In 4KH, when they first arrived on Earth with the appearance of 14/1(7?) year olds, they were about 4 and 6 respectively. Now, they're about 6/8 respectively, and have the appearance of being in their mid-twenties. Yuu and Fo, their kids, are perhaps only a few months old, but have the appearance of toddlers, at about 2-3 "years". Like their parents, by the time they reach 4-5, they'll probably look a lot like Usagi + company.
Like plants (well, specifically trees), they have a relatively quick time of growing to maturity, but will continue to live for many years after that (so they won't be dying of old age at, say, 20).