I am 46 and often get mistook for being more than ten years younger, sometimes as young as mid or even early twenties. I think for a dude it can be possibly a worse situation than for a woman, in that you get less of the nice and flattering comments about “oh you look so young!” whereas instead your professional competence may be questioned and you may be mistook for an intern or someone junior when you’re really mid-career or further along. Obviously, it depends markedly on your career field, as well. I work in journalism with an emphasis on action sports and I’m also a coach of soccer and track and field. I’ve been mistook before coaching older teens in soccer for an athlete in the age group I coach and have also been mistook coaching college track for a collegiate athlete instead of a coach. But because I’m into sports like surfing and skateboarding and I work associated with these sports, I get to wear surfwear and skate-wear most every day (boardshorts or jeans, sneakers, T-shirts). That and the fact I’m think are key reasons people presume I’m much younger.
An advantage is, as a skateboarder people are not asking me “why are you still riding that silly skater-board when you’re over forty?” that much since they think I’m younger (but news flash: the legendary Tony Hawk is 50 now and still shreds). To people who don’t take me seriously at first per how I dress or how young I look or my bleached blond hair, I’m just like “meh”—doesn’t matter, because people I work with do know and respect me. The fact I work a lot with Koreans as a journalist and translator also means that my youthfulness there is, as opposed to with Americans, taken as something of an asset because youthful looks for men are more-prized in contemporary South Korea.
But you do have to be able to bring the gravitas, seriousness, and tenor of character to show you’re responsible and intelligent if you’re going to be a professional and look youngish.