I'm 32 and still get carded all the time unless I haven't shaved for a few days. As long as you're relatively trim, have all of your hair, and no gray hairs...a lot of people will have a hard time pinning you down.
What really surprises me is how old some people look. There was a guy on our project...really nice guy, but he was very overweight, balding, and already going gray. I assumed he was mid 40's only to find out he was a couple years old than me (~34 now...32 when I met him). That would suck.