A child starts gaining bladder control around 18 months of age.
However the child's bladder control is still not great until they are around 3 years old and even then it can still be weak.
A child's bladder control is not at complete control until they are above 5 years old and some kids may still have a weak bladder and wet their pants sometimes until age 8 years old.
Some kids keep their pants dry at 5 years old and some kids don't keep their pants dry all the time until 7 or 8 years of age.
Toddlers below 18 months of age have no bladder control or bowel control.
A baby has no bladder control until at least 18 months of age and even then at 18 months of age the toddlers bladder control is still weak.
It's not until the toddler is around 2 and 1/2 years old to 3 years old when they can gain full bladder control.
Some kids even have weak bladder muscles until 6 to 7 years old.
Although a toddler can be potty trained at 18 months of age it does not mean they can get to the toilet in time to pee without peeing their pants.
Toddlers really should be in diapers until at least 2 and 1/2 to 3 years old until they can hold their pee in without needing to pee every 20 minutes or so.
If a toddler can keep their diaper dry for an hour or longer then they have bladder control but if the toddler has a wet diaper within 20 to 30 minutes and pees every 20 to 30 minutes then they are not ready to potty train.
A child should be able to know when they have to pee or poop and then go to the toilet on their own and pull down their pants, use the toilet and then pull their pants back up.
Until then they are not ready and should be in diapers.