What age to start hiking? packing?

[Duck-Slayer]

So is there a reason too not pack till there older? I mean if you keep the load light not sure that it will impact there spinal column and such? my goats will be 18 months this fall, I would like too go and backpack for mule deer this fall in some high country here in Idaho, I have 3 alpines, would it be ok too say put 12 pounds or so on each? that way they can at least carry out the deer for me and I can carry camp?
Matt