I've heard that part of the reason for the longer time for the NFL vs the NBA is actually a case of player safety (not saying those making the case don't also have a self-serving benefit from doing so, just saying that it's a reason I've heard articulated).
The basic premise rests on two facts: 1. the human male musculoskeletal system, and especially the tendons and ligaments in the major joints, does not reach full size and strength until sometime around 24-25, and 2. football is a much higher impact sport, especially at the pro level.
#1 is a medically true fact - it really can't be disputed. Yes, you can post a picture of an 18 year old that's 6'2" and 240lb of all muscle and ask how that's not a "physically mature male," but that's not medical evidence. The simple fact is that the young man in the picture will likely get to be another 1/2" to 3/4" taller, and his clavicle will continue to slowly grow (broadening the shoulders) until he's 24 or 25. But, more to the point, even if his bones don't grow another millimeter, and his muscles are as big as they will ever be, the tendons and ligaments that hold those two things together won't reach full strength until he's 24 or 25.
So, I can see that there may be an argument that before we'll hire you to be hit hard and thrown to the ground by a bunch of very large and very fit men for 3 hours a day 20+ times over the course of 21 weeks every fall, we want to be sure that your body is as best able to withstand that as is possible. There is, most likely, a marginal increase in player safety as a result of the policy.
How big that margin is, I don't know, and I don't think anyone else really does either. Yes, the player's joints, tendons, ligaments, etc are stronger when they are 24 than they are when they are 19. Yes the game is more physically demanding in the pros than in college. Whether the added musculoskeletal strength adds any meaningful protection against the additional physical punishment is probably an open question, and not one with an easy answer: sure, there's probably some level of increased injury (a weaker joint is a weaker joint), but what is the level and how do you compensate for a younger body healing faster after injury?
It's not like we have controlled, random sets of young football players and can send some to college and some to the NFL to conduct an experiment - we don't even have non-random populations to test against each other, so there's no ideal data on which to base a judgement. And, for better or worse, the only actual data we do have, the medical data that tells us that human male musculoskeletal systems don't reach maturity until close to 25, points in the direction of an older minimum age for entry in the league.
