How Old?

Overheard at Redmoon Theatre's Winter Pageant while the aerialists were twirling above our heads...
Mom:  "Those girls don't look old enough to be able to do that."
Brunette Twin:  "Right? They look about Sissy's age."
Mom: "How old do they look?"
Brunette Twin:  "About Sissy's age."
Mom:  "Baby, you're twins. If they are Sissy's age, they are your age too."
Brunette Twin:  "Oh, yeah. I guess you're right."