How many years has Mo Farah's final year as an athlete taken?

Should have stopped after his last gold. 

I never understand this call for once successful sportspeople to stop competing when they are past their prime. If they enjoy competing and are good enough to be allowed to participate, why not let them enjoy their active sports career for a few more years? They are not doing it for you, they are doing it for themselves.

I think Calvin's Dad it is because some people who have never been good at sports enjoy seeing people get older 

As you say, if the individual likes competing then they should be encouraged to crack on