Although he was supposed to be 39 years older, Ronnie Barker (Arkwright), was in fact only 11 years older than David Jason (Granville).
In the pilot episode Granville's age is revealed as 25. However in the actual series he is 19.
There is a running gag throughout the show where Mrs Featherstone doesn't like to be served by or even to spend much time with Granville, as a woman of her age has nothing in common with a lad of his age. In real life, Stephanie Cole is a year younger than David Jason.