I don't know; I don't see 26 year olds as "kids". Both men are grown men, that the 26 year old is in school; I was in school when I was 50+, so what? I would not worry about what your boyfriend does in/with his life unless it directly impacts on you; if you were living together and he went out with this guy after dropping you off or something, that would be different. Whether the 26 year old pays your boyfriend the money owed is your boyfriend's problem too; if your boyfriend owes you money, you can then insist he pay you and lay down boundaries but whether they are "friends"/drinking buddies or not doesn't bother me, other than what it shows about each of them and whether I want to be with them, etc.

If you do not like your boyfriends behavior, you tell him and then you have to decide how it impacts you, if you dislike who he "is" as a result (if I were to dislike something my husband did, he'd seriously discuss it with me, think about it hard, and change his behavior or not depending on what we determined together; if he decided to continue with behavior that bothered me, it would be up to me to decide where my boundaries were, whether he was the sort of man I wanted to be with, etc.).

That you raise the "jealousy" flag idea is good; I'd think about my own behavior, thoughts, feelings, etc. and why I am that concerned about someone else's behaviors/how they're living their life. As long as he does not bail or come over on Saturday too tired to do anything or hung over, what do you care?