I have two boys and one on the way. It is proven that boys take longer to potty train and it's usually best to start later. Girls usually train at age 2years-2.5 years. Where boys can train earlier but it is easier on you and less frustrating for them if you do it between 2.5 years-3 years of age. That being said, the correct answer is you potty train when they are showing the signs. Below is a checklist that I used to help guide me when potty training my boys. I trained both my boys at three and they got the hang of it in a day but were trained in a week. There is no awards for the mom who trains their child first and if they aren't ready and you try, it will probably take you twice as long to train. Please make sure they are really ready. I always started with a potty training video, had my kids walk around in their underwear or naked and kept the potty close by. Take them every 10-15 minutes and make them sit on it. I had a potty dance we did every time they peed on the potty. They loved the praise more than candy so we just went with it!
Physical signs
Is coordinated enough to walk, and even run, steadily.
Urinates a fair amount at one time.
Has regular, well-formed bowel movements at relatively predictable times.
Has "dry" periods of at least three or four hours, which shows that his bladder muscles are developed enough to hold urine.
Behavioral signs
Can sit down quietly in one position for two to five minutes.
Can pull his pants up and down.
Dislikes the feeling of wearing a wet or dirty diaper.
Shows interest in others' bathroom habits (wants to watch you go to the bathroom or wear underwear.
Gives a physical or verbal sign when he's having a bowel movement such as grunting, squatting, or telling you.
Demonstrates a desire for independence.
Takes pride in his accomplishments.
Isn't resistant to learning to use the toilet.
Is in a generally cooperative stage, not a negative or contrary one.
Cognitive signs
Can follow simple instructions, such as "go get the toy."
Understands the value of putting things where they belong.
Has words for urine and stool.
Understands the physical signals that mean he has to go and can tell you before it happens or even hold it until he has time to get to the potty.