As young adults are in base game as a non-optional life stage (unlike in The Sims 2), it is like in The Sims 3 where sims who are young adult, adult, and elders and enroll in university. However, in The Sims 4, in order to enroll in one, your sim has to send an application and get the results (which can be done when teen is close to aging up). Also, there are scholarships that can be only be obtained by teens and young adults.
While at university, sims age normally (unlike in The Sims 3). The end of university won't end your sim up.
So in short, forget The Sims 2's young adult stage in The Sims 4.
I have also made a video tutorial about the expansion pack.
https://youtu.be/w7CHXZ2FP6s