honestly, college is so much more different then high school. you will see SO many different kinds of people. there will be people who look like they're younger. there were people in my class who looked younger and some even had braces still and one guy in my class was 17! don't worry too much about looking younger. hell, don't even worry too much about being "different" with a high voice, etc. as a guy because you'd be surprised.
as for the bathrooms, i highly doubt you'll end up not using one at school. the reason? the schedules are so much more different. during one semester i would have classes in the morning, have a 4 HOUR break then have another class that was 3 hours long after the break. my suggestion? see the colleges policies on LGBT things (contact them, they will also direct you to someone who could help you, you don't need to tell them your name), or contact the LGBT groups at the college. you might not want to "out" yourself, but they definitely should be discreet about it. they will definitely help you figure out who to talk to about it.
also maybe email your professors that you prefer a certain name because of the attendance, which they do take. they might not take attendance depending on how big the class is (my class of 30 attendance was taken, my class of 80+ the teacher would just pass it around).
i can just tell you that when i went to an adult school to get my high school credits, i wasn't on T or had surgery yet, or even my name changed. i talked to the guidance counselor and she put the right name on the attendances, etc. i had no issues using the washrooms, etc. the best thing you can do is ask. i passed no problems (i was stealth 100%) because i had a masculine chosen (now legal) name, looked masculine even though i had a big chest and had to wear a binder and wasn't on T. i wore sweaters in the summer because of my chest but no one said anything because i saw others doing it too. i think we overthink these situations because we don't wanna "out" ourselves or think people are accommodating and we are worried about violence or discrimination, but you'd honestly be surprised. it's also much easier for someone who is a trans man to pass and go stealth and not get hassled as much.
just try not to worry about what others think of you. plenty of cis guys look young (the chronic baby face is strong in my family) and don't worry how all the other guys act. you don't need to act like them to pass. maybe being a little more feminine can draw attention to you, but honestly, the worst someone would think is you're gay (Trans usually isn't the first thing people jump to). but as i said..college, lots of different people..lots of out and proud LGBT people. college is when you "find" yourself, so people are definitely one of a kind.