You can begin potty training your baby as early as age 18 months of age and sometimes babies will be ready to toilet train and be out of diapers by 18 months of age.
If they don't seem interested in potty training at 18 months of age you can wait until the child is 2 to try again with potty training.
A child should be fully potty trained and out of diapers during the day by age 3 and if they're still not potty training by age 4 years old then you need to push the issue.
A child will usually start taking off their own dirty or wet diaper or tug and pull at the diaper or go hide behind things when pooping when they're ready to potty train.