I don't have as much information on the law school as the medical school, but if Marius--who strikes me as a diligent but not particularly, er, zealous student--could pass the bar within a few years of leaving Grandpapa's, it's certainly within reason for Enjolras to have become a lawyer at some point between 1828 and 1832.
Bear in mind that not all of them necessarily started post-secondary education the instant they turned 18, that Joly might easily have had the same attitude towards medical study as Bossuet did towards law, and that their parents might well have wanted them to come back home to the provinces after they completed their studies--further incentive to drag out school as long as possible.
As to Combeferre, general perception seems to be that he's a well-rounded scholar including the humanities, but I think Hugo paints him as much more of a hard-sciences boy. Yes, he considers the impact that scientific advancement will have on society, and ponders questions of progress and revolution, which must necessarily include history, literature, Enlightenment philosophy, and maybe even theology--but I think he's a technical geek at heart.
[Dieu] entend ta voix, ô fille des hommes! aussi bien que celle des constellations; car rien n'est petit pour celui devant lequel rien n'est grand.
- George Sand, Les sept cordes de la lyre