What walks on eight legs until the age of one, four legs until the age of twenty, and two legs after?

Answer 1

Probably twins with attached body parts.
They crawl with 8 legs, walk on 4 legs and after they are seperated (usually seperated after 20 years of age) they walk on 2 legs.

Answer 2

The Weasley Twins!
First they crawl,
Then they walk,
Then Fred dies.