I am 20 and my little sister is 17 (how quickly time flies!) - I never said I didn't believe in Santa because I was afraid if I did he would stop visiting our house!
But every year we still bake a batch of cookies for "Santa", it's fun because really we all know but nobody says it.
Last year on xmas eve we forgot to make the cookies until later and my Dad chipped in with a worried look "Aren't you going to make cookies for Santa?!"
And we did!