Our DS is 4 1/2 months old. He's a bit above the weight/length curve but not by much. He's not yet crawling but seems to be champing at the bit to do so. He holds his head up well and has the strength though not the balance to sit. He loves being held up by his shoulders in a standing position, holding his own weight, and even to jump with some help. Is he too young to carry around in a child carrier backpack?