When I was 22 I dated a guy who was 19 and I found him to be more mature than the 27 year old I dated before him. Age is just a number, and in no time at all she'll be out of high school.
You both said you like each other as more than friends. My guess is that the reason she wants to take things slow is probably more related to the physical aspects of your relationship than anything. You're older, and so she probably has preconceived notions about the things that older guys expect from girls they're dating. If she wants to see you all the time and has introduced you to her parents and such, I'd say that you're the focus of her romantic interests and that there probably aren't any other guys in the picture.