Instant Adult Dramas..?

A lot of dramas begin with the leads as children who become adults in a few episodes. There are also dramas like I Love Lee Tae Ri and BIG where a teenage boy suddenly changes into a man for most of the series. Other than that I'm not quite sure what you're looking for.

There are a ton of historical dramas that start that way such as Iljimae, The Moon that Embraces the Sun, but many of these dramas including those two I warn end sadly D:

I'm not sure if it's exactly what you want but most of these consist of transitioning into adulthood.

-Go ahead (cdrama)

-Dear Mayang street (cdrama)

-Reply series (kdrama)

-A love so beautiful (cdrama)

-when we were young  (cdrama)

-Le coup de foudre (cdrama)

-my youth (cdrama)

-Age of youth

-unforgettable (2016 kmovie)

-because this is my first life (kdrama)

-be melodramatic (kdrama)

hopefully, this helped :) You can also check out the tags to some of the recommendations