It is a constant battle with my two. DS1 has dyspraxia so finds using a knife and fork really hard and has to be constantly reminded to use them properly. He is 7.
DS2 is 4 and could do better but we often find he is using his fingers too. I sometimes think he is doing it to copy his brother though.
I think by the age of 4 or 5 they should be able to use cutlery more often than they use their fingers though. However, it is a gradual process and they don't get expert overnight.