Question about age and looks?

Im only like only in my freshman year almost a sophmore and i've had people tell me i look 17 years or 18. I had a guy once that thought i was in college and was like about to hit on me then he asked me how old i was and i told him im like um im a freshman and he's like oh i didn't think you were you dont look like one at all! So i kinda took it as a compliment, but it wont be good if i look older when im like 20 and people will be thinking im 25 so im not sure as how thats going to go