Well to me it’s about trust. Dating friends is messy but it’s also how some of the best relationships are formed. If you is truly your best friend and you trust her, tell her how you feel. If she is really your best friend she won’t intentionally stomp on your heart.
I’m going to be blunt but you may get friendzoned. Girls (and guys) aren’t obligated to reciprocate feelings. The most you can do is just tell her how you feel and respect how she reacts. If you do get friendzoned and she does like someone else and it hurts too see her with him, then you should remove yourself. I won’t tell you, you need to be supportive and be there for her because that’s masochistic.
Now for the brightside. You can tell her how you feel and maybe she will see you in a different light. As human beings, we like being liked and thus we like people who like us. If you tell her how you feel maybe she might see you differently and develop feelings for you.
In the end, it’s your choice; you can either choose to trust her or regret never trusting her.
Wait till you are 18 (assuming that’s the legal age in which it is no longer statutory rape). As a guy who is also 21, I would also not think it’s a good idea to be dating someone who was 17. You just never know who is going to cry wolf when times get sour.
No need to rush into a relationship. If you like him and he likes you, a couple months is nothing. Just stick to being friendly for now and if something more comes out of it in time, then great.