I’m in my late 20s, but look way younger than I am. Because of this, many of my clients and sometimes even coworkers don’t take me seriously. How should I deal with this?

I can kind of emphasize with you as I used to face similar issue too. But maybe not as bad. Dressing and grooming as others have suggested does help. Next is also how we behave in terms of how we speak and carry ourselves matters too. At times when clients comment on me looking very young, I will smile and say Thank you for the compliment and continue to speak in a calm and confident manner. Usually after a while, they know that I could assist them, they will trust me. For some more friendly clients, I will still say thank you & will joke that unfortunately not really that young anymore. Even with colleagues, i behave the same way. Over time they also learn to respect and trust me more. At times, in meeting really demanding clients, we might not be able to do much. We can try to speak to the client patiently but if the client insist of seeing someone more "senior", just don't take it too personally. Now that I have reached a certain age, I actually start to appreciate this "look younger" issue. hahaa

For guys - Grow facial hair if you can

For ladies - make up does wonders

colored hair might also be associated with younger folks. Keep it black or dark brown

you can tell them that older workers tend to get retrenched first or hard to find job. Young is good.

For guys: tailored clothing can make a big difference. Go for mature hairstyles. Ask your friends if you look older with or without glasses, buy glasses or contacts accordingly. If u really feel there is a need to, get glasses eith no degree if glassea make you look more mature.

For women: longer hair makes you look older, so grow it out. Learn how to put on makeup - lots of tutorials for that. Go for officewear, tailored if possible. Wear high heels. Same tip about glasses.

For either gender, be confident. Fake it till you make it. Dont betray your consternation when people mistake you for an intern, just smile and inform them confidently that you are the manager (?) And u can assist them. Over time your poise and confidence will grow.