Funny because, I was kind of dating an Asian girl for a while (a legit Chinese girl here as an international student) and she always told me I looked like an old man. Whereas all my life, everyone else has always told me I look young. I shaved my goatee off once in the time span that we were seeing each other and she then thought I looked like a child.
Definitely agree that wearing "grownup clothes" and having a grownup hairdo is a must. Be conscious of how you speak, too--the more you speak like your students to make them comfortable around you, the most they'll perceive you as a peer. This includes using lots of slang and ending sentences with an upward inflection, making statements sound like questions. Older people speak with more authority.
I teach college students, and I could get away with wearing jeans most of the time if I chose to, but I like to wear my grownup clothes to command a little more respect, since I look younger than a lot of the other professors.
It's super easy to look dowdy. Shop second hand and stock up on shoulder padded high waisted strangely colored and patterned business suits from the 80s. Get a really unflattering single length bob, not a fashionable one, an unflattering one. Color your hair and then wash it till it gets brassy. Wear unnecessary and geeky accessories like a a lanyard (if you have no reason to wear one, make one, wear a highlighter on the end of it) and an old school cel phone that you wear on your belt. Get unflattering, not even ironic framed glasses, make sure all your costume jewelry is not actually fashionable to anyone under the age of 35. Most of all, don't smile, look severe and ready to mercilessly correct the hell out of someone's essay. Don't laugh at people's jokes. Carry around a coffee mug with you always. Wear ugly orthopedic shoes with socks and thick hosiery and skirts that hit below your knee and especially calf length.
OMG, I could help you so much if I were your personal stylist.
“If you want to tell people the truth, make them laugh, otherwise they'll kill you.” ― Oscar Wilde