I understand the answer,
A baby walks on four legs (crawls), it's weak.
A toddler-middle aged person walks on two legs
A elderly person walks on two legs and uses a cane
However, I think this should be rephrased. Morning, afternoon, and evening have absolutely nothin to do with it. It should be,
"What has four legs in the beginning, two in the middle, and three in the end. When it has the most legs it's the weakest."
Anyway, that's just my opinion. Other than that one part, it's a good riddle!