The term "sugardaughter" is typically given to females 18-25, however, this is arbitrary and can apply to females in their mid to late 20's as well.
In other words, If people assume your girlfriend is your daughter when they meet you and her---> she is your sugar daughter.
The word can be one word or two: "sugar daughter" or "sugardaughter" are fine.
"no no no. SUGAR daughter. She's old enough to be my SUGAR daughter"-Bill