How can Ric Mayell be Greg Davis's Dad?

He's perfectly cast. Greg has always made jokes that he looks like a giant version of Rik Mayall.
Such a shame to have lost Rik.

Stop being so literal and use some imagination. Also learn to spell MAYALL, you wouldn't misspell JESUS would you?!?! and hes the comedy messiah.

In "Good times" John Amos, who played James, was only 7 years older than Will smith, who played his son, J.J.

Rik Mayall seemed to age terribly after his quad bike accident. He only lived into his 50s & suffered from epilepsy after the fall.

Anyone who can't spell RIK MAYALL should be eaten alive by rabid dogs.

It works if you don't check their ages and assume that he must be old enough. I always remember The Golden Girls TV show where Bea Arthur (Dorothy) was actually a year older than the actress who portrayed her mother (Estelle Getty) on the show but at the time I didn't notice.

Because it's a fictional series, and there's no law to say that actors have to play characters that are the same age as themselves?

It makes as much sense as parents of 5'11 and 5'4 having a son of 6'8. And a teacher never being disciplined for swearing at children. And a father never being referred to a counsellor for attacking his son. And Jo managing to be well groomed whilst living in a tent. This is not real life; choices were made for laughs- not realism.