First of all, you should know this is mainly a slice of life manhwa.
The romance story itself isn't the main plot of the story. If you're looking for an intense shoujo romance, this is not the one.
Still, you should try it because I believe it captures the attention of any reader.

Should people stop living in a carefree way in order to become adults?
And when exactly are we becoming adults?
Nineteen, Twenty-one revolves about this interesting matter.

Yun-lee is a 21 year old girl who lives her life in pain due to the fact that she has missed one entire year of her life and realizes that she can't start over.
Ju Dong-hwi is nineteen year old boy that doesn't want to grow up yet, knowing he'll have to leave behind his youth days and become a responsible adult.
These two young lives cross paths because of a strong mutual interest in..cats.
Suprisingly this is what creates a beautiful friendship between them which causes each one to learn from the other.

The characters were also very entertaining.
Normally with shoujo you have people falling in love based on one aspect (either looks, money, position, personality etc.) but in this manhwa, the two bond over a common love for cats.
Dong-hwi is a very cheerful, optimistic guy, the kind of person that you'll want to be friends with.
Yun-lee, on the other hand, comes across as a very mature girl but in the end she learns much more from her friendship with Dong-whi as she begins to know how to appreciate the little things.

I think it's amazing how the manhwa author took such a random topic - the stray cats - and turned it in the main rising point of the story.
Actually, he seems to compare the cats' lives with the life of humans searching for the balance between these two paths.

The Art is full of details, pretty realistic and in color.
It's gives a relaxing feeling that soothes this mild love story.

Overall "Nineteen, Twenty-one" is a very unique, sweet, plain, good love without the cliches and third-parties you're used to in many shoujo mangas and manhwas.