The answer is Man. Crawls on hands and knees as infant, walks on two legs for the majority of life, then uses a cane in old age.
This is actually a very old riddle. There's an excellent story behind it.
EDIT: Removed potential spoiler
I am fully aware of both the story and the answer to the riddle, I was cheekily nodding at Arc because of the comment he was responding to.
Ah, I kind of lost sight of the original post when typing my reply. Edited previous reply to remove potential spoilers, assuming my theory is correct.
