Trust me, in about 20 years, you'll be grateful for your "baby face." But I grant you, it is frustrating in the meantime. I'm 53. I just started going to a new dentist and had a slight disagreement with the dental assistant who was taking my information. She was sure I was about 44.
I used to french braid my hair and tuck the long braid portion up under the part that was braided down the back of my head. It stayed in place with one hairpin. Or a bun. Never could figure out how to get a twist to stay up. Ponytails are out, I'm afraid, except in your off time.
A little makeup should help.
But clothes help the most. I started wearing suits at one point, not because I liked suits, but because people took me more seriously when I wore them. These days, maybe not a suit. But a jacket, a button-down shirt and dark wash boot cut jeans. A button-down shirt and a pencil skirt. A twin set (cardigan and shell) and nice slacks. Check out what the current fads are and don't wear them at work.
In general, I have found wearing a top and a bottom plus another layer adds a bit of authority. So tee shirt, khakis and a cardigan. Button-down shirt, jeans and a jacket. Try adding that top layer to some of your favorite outfits and see if that helps.
Also, look at your jewelry. For the times when you want to project a more professional image, avoid the costume jewelry. No big, dangling earrings. Get a pair of pearl studs and some simple, basic gold or silver hoops. Maybe a small button earring. But make sure they either are or look like gold or silver and real gemstones. A wristwatch. Only one ring, in addition to your wedding set if you have one.