Chalk me up in the "been there, done that" category.
As said before, make sure the two of you are on the same page. And it is tough to remian just FWB with someone like that. Depending on your sport, you aren't friends with your teammates, there's something more. There's already a trust relationship, and add in sex, then there's a recipe for an emotional attachment. That doesn't mean the two of you can't make it work, but it won't be easy.
I hooked up with a teammate in college for the better part of three years (until he graduated and moved on). It was a really good experience for the two of us. In no way do I regret it at all. That being said, it could easily have blown up in our faces and had consequences for the team as a whole (which would have been unfair to them).
#1) it won't be easy keeping it from the team. People can be more observant that one might think. But you can do it, especially if the team isn't especailly close or live together.
#2) The fact that he is younger and less experienced doesn't help. He is trying to find his way in things and might not be ready to accept his sexuality just yet. Maybe he is just experimenting, maybe he is bi, maybe his is gay. Only he really knows and he has to figure it out, but it might put you in a tough situation. What happens if he has a girlfirend then comes to you one night for a hookup? Remember that he is in a different place in his sexual development than you and many times that can cuase problems. Many time the guy in your position just doesn't want to deal with the issues he is dealing with as "you've" already gone through it, and it was painful enough the first time.
#3) The age difference might be a factor. At 18-19, most guys aren't all that emotionally mature. But by the time you get closer to graduation, many guys, if not most, are now emotionally pretty close to being men, as opposed to being a boy with a man's body (like when you started college).
#4) A lot of thie problems can be dealt with if the two of you sit down and really talk it through, but that's easier said than done with a 19 year old (who probably is more interested in just stripping off his clothes and dealing with the consequences later).
Best of luck.
Please feel free to ask for advice. That's what we are here for.