Some kids, such as my middle child, for example would stay in diapers until college if you let them. Once they turn 3 and show signs of awareness (like telling you he's going in his diaper and asking for the potty) then you know they are physically ready.
This is what I did with my middle child when he was 3.5 and still not wanting to use the potty:
I told him that he was a big boy, and they don't make diapers for big boys. I had him watch me throw the diapers away. We pulled the underwear out of the drawer and he picked the pair he wanted. We went downstairs and got the kitchen timer out. We filled his sippy cup with lots of fruit punch (his favorite juice), covered the couches and carpet with towels and got to work! The timer would go off every 15-20 minutes and we'd have to go up to the bathroom. He didn't like the timer and would resist, but I didn't give him a choice. I kept reminding him of all of the things he couldn't do until he went potty (play in the playroom, play on the computer, play outside etc..)
Two days, and a few accidents later he was trained. I used the same method with my youngest and it worked just as well. It only took one day with him.
Best of luck!