I'm not sure it's all _that_ unique. Elephants are physically mature at 15 to 20, say, so not that different to humans. Other apes are also similarish to humans in this.

Many cetaceans show similar dependency on their parents. They're also some of the few species where the females undergo menopause, like humans. (Elephants might have menopause, too.) Perhaps not coincidentally, maternal elders are very important for these species, often helping their children and grandchildren for decades after they are born.