Like everyone else said, it's a matter of personal preference, and I think that "age rules" can only show you a different direction you may or may not want to tread. I will be 40 next year, and I know that as a petite with shapely legs I look nice in minis, but still I got tired of them and purged them all (although I still have one or two dresses that do not reach the knee and wear shorts in the summer a lot). My sister, only one year younger, wears minis with heels all the time. She manages to look tasteful with tops, hair and make up, and no one resents her for dressing "young" in her late thirties. The way I see it, though, if you "act" young you will be considered young, and I just don't want to be so young anymore