Define dressing young. If you want to be taken seriously, never wear anything that says "pink" or "juicy" across your butt.
If you want the Dammmmm ! reaction, show up in his bedroom unexpected, wearing nothing but a naughty grin and one of his dress shirts, with the top 3 buttons undone.
If you want to impress him with your fashion sense, just go one step up the formality ladder for whatever activity you're doing with him. Only one step. You don't want to be grossly overdressed. Keep the stuff you wear understated, but of high quality, well fitted, and chosen to compliment your shape and color. Don't over accessorize either like some kind of Olsen Twin wannabe, and don't overdo the makeup or perfume. Understated class.
Your boyfriend will take you seriously depending on how you communicate with him. It has nothing to do with clothes. It has to do with maturity. Personally I take everybody seriously. Even children I consider them as young adults and I ponder on the meaning of what they say. If I have a girlfriend in her early 20s .. I would take her seriously as I would take a girlfriend in her 40s . It depends on his personality and it depends on how you communicate .. you should be confident and assertive.
23 is young, hon! How do you like to dress? There's ways to look older like him, like tucking in and/or wearing blouses/business-y styles, but if it's not you and what you like, you're gonna hate it & it's gonna show! Besides, I agree that I don't take or not take girls seriously because of their clothes! Just dress as & be yourself, & wear what you like best, & the rest will take care of itself!
Blazers are always a good shout. They finish an outfit off perfectly, and there are so many different kinds of blazers. I'd say go for boyfriend ones, or longline ones when you want to be all casual, and save the fitted/tailored ones for work or something. Skinny jeans, and blouses/shirts are quite grown up, and accessories always look good, if you don't go over the top. E.g. if you're wearing small ear studs, you can wear a long necklace, a chunky bracelet, OR some rings. You wouldn't wear studs, the necklace, and the rings, that's just too much. Also, a nice stylish watch helps make you look classy and older.