Grownup (noun)

1. An adult person.
2. Someone who has grown to full maturity.

Origin:

19th century; earliest use found in the christian advocate (new york). from grow up + -n.

Examples:

1. She was surprised when her little sister became a grownup.
2. He didn't like acting like a grownup.
3. The children were excited to play with the grownups.
4. He was still a child in a grownup's body.
5. The party was for grownups only.