I've got the opposite problem - I look older than I am. I was rarely carded once I turned 18
and everyone at work thinks I'm around 30 when I'm actually in my mid-20's.
It's all in how you carry yourself. If you order a drink or buy some booze like it's no big deal, you won't get carded. If you dress well (nice shirt and slacks/skirt, no jeans) you'll look older. If you wear your hair short or up in a bun, you'll look older. If you stand/sit up straight, you'll look older; if you slouch, you'll look like a kid.