Adjective - A phrase indicating that a girl who is traditionally jailbait is built like a girl who has reached the age of consent. It usually indicates she has a fantastic body, great breasts, amazing legs that go on for days, or an ass that just won't quit- fantastic enough that one wishes she was already 18, and mature enough looking that one would not feel any moral quandary should they have intercourse with her.
Question: Did you watch Mad Men last night? That girl who plays Sally Draper was looking fine as hell. I'd love to tap that. Is she eighteen?
Answer: She's eighteen enough.
Reply: Hell yeah she is.