Movies: Wait 'Til You're Older (2005) starring Andy Lau

sypnosis of the movie (credits to HK Entertainment News in Review):

(Official synopsis) Twelve-year-old Kong is an unhappy child. He has not been happy since his mother's suicide three years ago, which he blames on his father and stepmother. He runs away from home one day and meets an eerie old man who claims he has a potion that can speed up the life process. As much as he would like to believe it, this is all baloney to him. So when he accidentally taints his blood with the potion, he hardly expects his fate is about to change forever. It is an impossible dream come true when Kong wakes up the next morning to find himself already a twenty-year-old young man.

Directed by
Teddy Chan

Writing credits (in alphabetical order)
Suk-Yin Chan
Chi-Kwong Cheung

Cast (in alphabetical order)
Nicola Cheung
Xiaogang Feng .... Bum
Ka Tung Lam .... Vice-Principal Chow
Andy Lau .... Kwong
Bingbing Li
Karen Mok .... Tsui Mun
Kristal Tin
Chapman To
Felix Wong .... Chan Man
Cherrie Ying .... Miss Lee