Everyone is different; some people throw their hardest at 18 and others at 29. Generally, your "peak" will be somewhere between those two ages.
Sometimes all a pitcher needs is a new throwing program, or a slight tweak in mechanics, or a little more overall body strength, or all three, to add a few more MPH on the fastball.
From my personal experience I coached a college pitcher who topped out at 87-88 MPH when he graduated. He had great mechanics but was a stringbean, about 6'3", 175. Eventually he filled out to a shade under 200 lbs., gaining a lot of leg strength, and hit 92-93 MPH in his mid-20s (in MLB !).
I played against another guy (lefthander, too) in high school who was clocked at 83 MPH as an 18-year-old. He was already a big kid --- actually a much better football player, at 6'3, 230 --- but his mechanics weren't too great. He played three years of D1 football then decided to try baseball again. Well, someone must have fixed his mechanics because all of a sudden he was throwing 95-96 consistently. Today he is in the bullpen for the NY Yankees; Ron Villone will be the first one to tell you he was barely the third-best pitcher on his high school team!
So, it's possible you haven't yet peaked. I'd suggest finding a good pitching coach / instructor to assess your mechanics, either through your college or a pro scout. If your mechanics are good, you might be able to add some speed through a physical training regimen that includes strengthening your legs and your core (middle), plyometrics, and a structured throwing program.