I don't think it looks young at all. I would probably add a decorative white pillow for the bed. A faux or real green plant to the nightstand in the corner and or a few green plants to the window sill. Plants always make a room look more mature.
Move the pictures above the headboard way down. The bottom of the frame should sit about 5 inches above the headboard.
The lamp by the desk should be to the left of the desk. Switch the peg board and whiteboard. The pin board is kind of messy, so that should not be in the center of your room. Ideally, if you don't need, I'd get rid of it. But also, they are hung way too high. Move them down to where the bottom of the frame would sit just above the height of your laptop when opened. Then maybe you could hang something like this horizontally above the two boards.
Find a small lamp for your desk if you can. Something like this.
If you could budget like a hunderd bucks you could probably get a couple of plants, desk lamp, art, and pillow from somewhere like Homegoods, Marshall's, or Tuesday Morning if you have those store near you. Thrift stores might be a good option as well. And don't forget to check Facebook marketplace.