Ideal age to be out of diapers?

My boyfriend's half-brother is almost 4 & his parents still put diapers on him all the time. He'll use the potty once in a while at home but they still always put them on him (maybe this is more of the parents' laziness but I'm not sure). It seems a too old to me but what do you think is a good age for a kid to be potty trained or mostly potty trained?