The main leads in 55:15 Never Too Late revert back to their younger self just like 18 Again. They all have regrets that they want to try and fix.

It has a similar premise, though it is a romance between a man and a woman.

-heart transplant
-change in personality bc of their new heart
-gaining feelings for another person bc of their new heart
-rich/poor dichotomy
-rocky start, etc.

Amnesia, undercover work, and drug wars are some of the major similarities. The two shows are both action romance, although, I would say "Age of Legends" is more light-hearted.