You can potty train a toddler at 18 months of age.
Some toddlers are ready to potty train at 18 months of age and if so you can easily potty train the 18 month old.
At 18 months old some toddlers are fully capable of learning to use the toilet and can learn to keep their diaper or pull up dry.
However some toddlers aren't ready to potty train until 2 to 3 years of age which is okay as well.
There's no set age for a toddler or child to be potty trained.
So if your toddler is not interested or ready to potty train at 18 months of age then wait a few more months or wait until the toddler is 2 years old and try again.
If they still are not ready then stop with the potty training and try again when the toddler is 3 years old.
Just because some toddlers potty train at 18 months to 2 years old it does not mean your toddler has to be potty trained at the same age.
There's nothing wrong with keeping your child in diapers until they are ready to potty train.
Changing diapers is much easier than dealing with potty training and cleaning up wet pants, bedding etc.
One of my sons didn't potty train until he was 6 years old and eventually he just decided to stop wearing diapers and went potty on the toilet.
I kept trying to potty train him until he turned 4 and then decided that it was not worth the struggle so I just kept him in diapers.
Then one day a month after his 6th birthday he decided he no longer wanted to wear diapers and wanted to use the toilet.
This is a rare case but it happens and there's nothing wrong with any kid of any age wearing diapers as they will eventually get it and go to the toilet.