Phillies have a long history of low bonus signings who start out in the 88-90 range, and a few make big jumps after 2-3 years of "home cooking." A lot of LA pitchers are just plain skinny, probably due malnutrition (I suspect especially Venezuelan pitchers these days), you see some of them pack on 40+ lbs after a few years.
I think around age 22 or so the odds of a big velo jump diminish, improvements after that are more likely due to tweaking mechanics (a number of ML pitchers have added 1-2 MPH that way) than filling out their frames.
The other issue is short-term and sustained velo, a lot of these guys put up good numbers in short stints, but projecting them as starters requires getting some readings around pitches 60-80, because to pitch that deep into games, they have to learn to pace themselves so you get a "truer" read on velo.
I'd say these days a SP needs sustained velo around 92 or so, a little less if they have great movement and command, more if they have a "straight" FB. But as Suarez and Nola show, the margin for error is smaller at 92-93 than 95-97, but I've seen more and more hitters square up 100 MPH FBs - self-selection for quicker bats as average velo rises. Velo won't help a straight FB thrown in the sweet spot (middle of the plate between the knees and waist).