History shows peak velocity is developed at a very young age and pitchers don't normally gain additional mileage on their fastballs as they get older and their bodies mature. Velocity is a natural talent. No different than world class short distance runners. They don't all of a sudden at 15 show this incredible talent. They stand out a very young age like pitchers do with their velocity. Here are pitchers from different eras and when they got their major velocity.

Wikipedia  Dwight Gooden                                                                                                                                                                                                                      Gooden made his major-league debut on April 7, 1984 with the New York Mets at the age of 19. He quickly developed a reputation with his 98 MPH fastball and sweeping curveball, which was given the superlative nickname of "Lord Charles," in contrast with "Uncle Charlie," a common nickname for a curveball.   Wikipedia  Josh Beckett   Beckett is typically called a power pitcher. While many power pitchers evolve into finesse pitchers later in their careers, it has recently been noted that Beckett has already started to rely more on finesse pitching, but still possesses top-level velocity. His four-seam fastball ranges from 92-98 mph, and Beckett imparts heavy tailing action to move it through the zone. He complements this with a two-seam fastball (91-94 mph)   John Sickels   Josh Beckett   Rookie profile: Josh Beckett
By John Sickels
Special to ESPN.com By John Sickels   Beckett is the prototype young pitcher: all of his offerings are above-average, and he throws strikes. His fastball is the headliner: hitting 97 at times and clocking a consistent 94-95. The pitch also has good movement; it is not a straight heater that hitters tee-off on. Beckett was 21 when Sickels wrote this report.  
  From Yahoo Sports Brien Taylor Lord, that fastball. They swear it tickled 85 when he was 12. In his senior season, Taylor worked 88 innings, struck out 213 hitters and walked only 28. His fastball rested at 95 mph and often hit 98 and 99. Taylor was 19 at the time   From minorleaguenews.com  Joel Zumaya   Joel Zumaya is another of our diamond in the rough players. Plucked from Bonita High School in the very busy, for scouts at least, talent market of San Diego County,  his early looks by scouts were pretty ho-hum.

The routine reporting sent him into the 11th round of the draft, where Detroit picked him up at position 320 overall.

Joel throws heat clocked in around 98mph tops. Throwing is almost inaccurate. He hurls the ball with such intensity you think, at times, that either he or the ball might explode on release. This report on Zumaya was when he was 17.

Tim Lincecum - The Future
by Richard Van Zandt, BaseballEvolution.com
August 7, 2006

From the elder Lincecum I found out that The Future has a wide arsenal of pitches that include two fastballs (a two-seamer and a four-seamer that generally settle in at 92-94, but he hit as high as 97 on Saturday and has been previously clocked at 101). Lincecum was 22 when he had this velocity.

Bob Feller had major velocity at 17, when he struck out 15 in his first major league start.

The above pitchers reached their peaks no later than 21 but most during their teen years. That is why I don't feel Phil Hughes will be a major talent. 91 MPH, without a lot of movement and no late tail is not a fastball that will have a great deal of success. And history shows that his fastball will not gain significant  velocity as he gets a little older.