Mr. Back (2014)

Both delve into themes of death and similar themes of reincarnation
They both deal with an Ahjussi romance
Overall, similar fantasy-romance themes
Similar supporting character roles - Goblin's Yoo Deok-Hwa VS Mr. Baek's Choi Dae-Han

Older male lead becomes young
Tries to develop a relationship with his son (and his daughter in 18 Again)
The difference is that the romance is between his wife in 18 Again whilst Mr Back is with an employee at his company

The movie is about a father who dies but then gets another chance to bond with his son. He becomes a high schooler and tries to become his son's best friend. HILARIOUS!

Instead of an old man an old granny becomes young again due to some supernatural cause.

Though the plot is different, they both involve the older man turning young (with the same face they had when they were younger), developing a relationship with the younger female lead (brought together by fate and met before the ML turned young) and the male lead bonding with their son (with the male lead pretending to be their brother).