Here is the potty training readiness checklist for a 2 yr old , there is none for 18 months.
Can walk and 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 her bladder muscles are developed enough to hold urine.
Can sit down quietly in one position for two to five minutes.
Can pull her 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 she's having a bowel movement such as grunting, squatting, or telling you.
Demonstrates a desire for independence.
Takes pride in her accomplishments.
Isn't resistant to learning to use the toilet.
Is in a generally cooperative stage, not a negative or contrary one.
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 she has to go and can tell you before it happens or even hold it until she has time to get to the potty.
In addition if you start too early and he's not ready it will take longer to train him.