It really is up to you and your child. Every baby is different some are ready and showing signs of potty readiness as early as 13 month others as late as 2 years old. Deciding when you are going to start potty training depends a lot of whether or not you plan to stay home, the type of daycare you put your child to and whether you plan to do full day school. If you plan on doing full day school at 4, I wouldn't wait until 3 to start potty training b/c schools only take you if you are fully potty trained. As well, I've heard that many daycares do not accept kids who do not sit on the potty to go.
If you are a stay at home mom or you have a nanny or other caregiver at home then it's easier to potty train when they are older b/c you can give that one on one attention. Private daycares will help you to potty train your child if they are already comfortable with the potty.
The first thing to do is just get your child comfortable with sitting and going on the potty in a casual way. You can start from the time they are able to sit on the potty. You are not "training", just giving them the option, if they go make a HUGE deal! jump, dance, do a potty song! they'll want to do it again next time to see you acting like a silly mommy.
I never "trained", but I gave Katelyn the option to sit on the potty, I told her what the potty is for (even whe she only 8 months) when we go to the washroom I would show her that this is where the poo and pee goes. I would empty her diaper in the toilet and flush it so she made connect that this is where the poo goes. by a year she was only pooing at home in the potty and by 18 months she was out of diapers all together in the daytime. She was fully potty trained with no accidents by the time she was 2. Now we are working on night time training.
I know many other moms who had their kids out of diaper in the daytime by the age of 2.