It can't be used as a metric for young though can it? Especially in an industry of image. I'm not trying to insult anyone, or suggest he's bad looking. Many think quite the opposite, but it's hardly fair to say gray hair doesn't elicit associations of old age. It's one of the most noticeable effects of getting older. Coupled with male pattern baldness, which isn't exactly the same as voluntarily shaving bald, & yes it does evoke an image of aging.
Stewart is quite possibly the most confident man to EVER sport the male pattern baldness look, but he did look 50 when he was 44, & it wasn't just balding & graying that did it. It's his genetics.
This is Jared Leto at 47, who still looks 27, & a dark haired, full hair piece wearing Stewart at 35, with wrinkles, creases, furls, etc... His face has looked miraculously the same for 40 years, that is to say... not youthful