He showed interest off and on for about 2 years (a day here, a week there, no interest for months at a time, then a day and no interest again) before our Ds was actually *physically* ready to potty train. He was 3.5, and daytime trained in a week. Nighttime training occurred about 6 months later, and happened over a weekend. No stress in either situation.
The nervous system has to completely rewire before a child can have conscious control, no matter how much they WANT to be in control, so there's no use in rushing. Also, because the nervous response is tied to emotions, the fastest thing to take AWAY conscious control is fear/excitement/stress. Even happens to adults. It's why we always have to pee right before a job interview, even though we just went
While I have known one or two very very young wobblers (18mo) who were potty trained the majority of parents I know IRL who claim their wee ones are trained that young, their kids actually AREN'T trained... the parents are. They watch their kids like hawks and leap up to take them to the bathroom, or have them go sit on the potty every 1/2 hour. Which is parent-training, not potty-training. If you can't afford diapers, that's one thing, but true potty training is accomplished once you don't have to do a durned thing.... they go on their own without having to be constantly monitored.
A trick on night time training, btw, is the opposite of conventional advice. :) We did it on accident (our Ds is hypoglycemic, so we always gave him about 12oz of warm milk as he was sitting in bed)... but more and more parents I've been meeting are doing it on purpose:
Instead of withholding liquid, get them to drink a LOT right before bed. That way the pressure on the bladder is unmistakeable, and their sleeping mind learns really really quickly to wake up, because of bladder pressure and then being soaked to the sheets. It causes 1 or 2 really wet nights (a mattress protector, and a stack of sheets and towels and ready to go jammies right by the bed make a quick/calm change super easy) and *poof* night time trained.
It's something to consider though... do you really WANT your 1.5 year old wandering the house at night? Even if he was one of the teensy minority who's physically ready (and there's nothing special about being ready early, just luck of the draw), do you WANT them getting out of bed and heading into a room where it's possible for a younger child to slip and hit their head on a hard surface or to drown? And of course, that's if they only get up and use the toilet, and don't start playing alone in the middle of the night on their own. Those were both active considerations on our part before we started potty-training.