Yes, young adults can't get married. They can get engaged though (even to regular adults and elders), but be careful of those evil (cow) mascots and drama professors as they may try to attempt to flirt with sims. Even when that fails, it will be counted as getting caught cheating when the partner is there.
The only thing I can say is that there is no need to rush. If you really want to rush it, have them all drop out. But if you want them to graduate and have access to four degree-exclusive careers, it is time to slow down. When all exams are successfully passed, the college usually lasts total of 24 days. Just don't play with their parents a lot so they don't die before they all get married.
