This example will be very hard to beat for anyone who cares about historical accuracy.
In the 10 May 1980 episode of Fantasy Island My Fair Pharaoh/The Power the "My Fair Pharaoh" part had the protagonist going back in time to meet her hero Queen of Kings Cleopatra VII Philopator. Cleopatra's brother and enemy King Ptolemy XIII Theos Philopator (62/61 BC to 47 BC) was portrayed by Michael Ansara (born 15 April 1922, died 31 July 2013) who was thus aged 58 years and 25 days when the episode aired and was probably 43 or 44 years older than Ptolemy and 3.8 to 4.07 times as old as Ptolemy when filming his scenes.
This is an example of a middle aged man portraying a character arbitrarily depicted as an adult despite being one of the most famous child rulers in history!
So this could be the extreme example depending on whether you go by Ptolemy's historical age or his unspecified adult fictional age in the episode.