Ellis, it's very hard to look like an adult man or "grown up" when you're pre T. I passed 99% of the time, you know why? Because everyone thought I was a 15 year old boy. It would aggravate me because people would always call me "buddy" or "pal" and in the same breath address my male friends as "sir". However, I was so grateful to at least be recognized as male that I tried not to pay much attention to it. I've known a 32 year old transguy who was pre T and passed wonderfully, however it was because people thought that he was still in highschool. Funny thing was, when he told people his age, they tended to put two and two together that he was AFAB. I feel like you may have meant "mature" when you said grownup, and I found that using gel to slick the top over effectively made it look mature. However, that was a "mature" 15 year old boy rather than a "juvenile" 15 year old boy. Still not "grown up" per se.
This is just me trying to share my experience with guys who are early in their transition. I have been called a "bull dyke" by a cashier when I was walking out of the store. Might I add that I had a "round face" as well as hair the length of the sides of that undercut. It did not make my face look thin, it accentuated my feminine facial features and made me go from "boy" to "hyper masculine woman". Definitely not to "grownup". That being said, what worked for me might not work for everyone. Probably the most important thing is to have a haircut that you like, that makes you feel confident, that doesn't make you feel self conscious, and that helps you express your masculinity.
