Recommendations

Both are modern adaptations of classic novels (one French and one Russian) again showing that a great story can be translated through time and culture...because human nature doesn't change.

Both are inspired by Alexandre Dumas novels, showing that a great storyline remains great as a translation into another culture or time period! Yay for classic literature (even if it was pop lit at the time!)

Both movies are about a girl & a boy seeking revenge. Both movies star Horikita Maki. Both movies are based on famous tragic novels........................

Both movies are about legendary bandits of warring States period.......................................................................

Both are about a orphan fighter based on famous manga by Ikki Kajiwara.........................................................................

Both are battle of wits psychological thriller movies based on famous manga.
Both star Tatsuya Fujiwara & Kenichi Matsuyama as protagonist & antagonist rivals..............

Both are battle of wits psychological thriller drama based on famous manga.........................................................................

Although the plot is different from Age of Youth, a lot of the ideas are similar. In both shows, 5-6 adults are roommates where cry together, laugh together, fight together, etc. A prominent idea in both, is the process of their adulthood. The characters struggle with finding jobs, balancing their relationships, and so on.
They also share some of the same cast! Son Seung-won is one of the main characters in Eulachacha Waikiki and is a side character in Age of Youth. Shin Hyun-soo makes a brief appearance in Eulachacha Waikiki and is a side character in Age of Youth.

Both are movie series based on epic jidaigeki vengeance manga by Kazuo Koike............................................................

Both have female leads who fall for their 'brother'. In Prince William it is her brother's look alike but in 'That Winter, The Wind Blows', the female lead is blind and falls for her long lost brother came back who is actually a fake.

In both dramas a look alike switches places. Unlike many dramas where there is blood relationship between the people who looks alike there is none in both cases.

Both dramas receive messages from the future. The story and why they receive the messages are different.

Both are about a popular guy in school (who is oblivious/doesn’t care about him being popular) and who likes a shy girl with low self esteem (because others make fun of her appearance). Both of the male leads also like the girl from the start, and the ML and FL interactions are always awkward but still swoon worthy.

Sending letters to the past to change your future? both movies got your back
orange tho follows a Japanese high school setting while to my 19 year old is a Chinese youth movie set in their college days
both are sure to give you the feels tho

Both movies are visually very stunning, shot in very beautiful Japanese nature, forests...................................................