youngest

The youngest is the last-born child of a family. The youngest of three, my youngest, the youngest child.

Birth order gets talked about as if it settled personality, and the research is much weaker than the folklore, but the youngest being indulged is a stubborn family belief everywhere. It works as a noun on its own — how is your youngest — which is unusual for a superlative. Note youngest child, youngest son; and that the youngest of three is correct where the younger of two is, since English keeps the comparative for pairs.