It did mature them, but the point is that they were force to mature too fast, and, at the time, the teachers were not in the state of mind to help their students.
That's why they still had issues. Nobody helped Mercury after she was abducted by the Weapon X Facility, but Hellion. Hellion, himself, stopped getting advice from his own mentor, because she began to distance herself from the students, following the "busing". Again, with Hellion, he had abandonment, and trust, issues, and instead of helping him, they threatened to kill him several times.
If their whiny, it's because the X-Men stopped being teachers, as the circumstances dictated they become soldiers.