I think it depends on the person and the hair.
Sometimes nothing make you look older than a short, low maintenance cut. Sometimes long hair ages people.
Short isn't low maintenance for me though as my hair is wavy I need to style it more and have it cut more, whereas if it's shoulder length I can let it be wavy, or straighten it, or curl it, or tie it back depending on the level of maintenance I feel like at the time!
My mum has long hair which she has in a bun or plaited, light blonde rather than grey and she's 72. My mother in law is a few years younger and has hardly any grey hairs, has poker straight brown hair in a bob, which she's always had since her 30s. I tend to think older people look younger these days as they don't go and get a blue rinse perm from 50 onwards.