Aamir Khan was 51 when he played the role of a 19 year old boy in Dangal.
From Bollywood Life article
So Aamir Khan has done it again what he does the best. The PK actor has gone through a drastic makeover for his upcoming film, Dangal, in which he will be seen going through three different stages for he will play the role of a 19-year-old, 29-year-old and 55-year-old.
Below is the image of him as 19 years old.
Another example, Amitabh Bachchan was 67 years old when he played the role of a 12 year old boy suffering from a rare genetic disorder called progeria in Paa.
From Wikipedia plot summary,
Auro (Amitabh Bachchan) is an intelligent and witty 12-year-old boy with an extremely rare genetic disorder called progeria. Mentally he is twelve and very normal, but physically he looks five times older.
Though, in this case, the character he played was supposed to look like an old man.