The best age to potty train a girl is around 24 months old.
Although some girls are ready to potty train by age 18 months it's not always the case.
Some girls may not be ready to potty train until 24 months of age.
Usually girls are ready to potty train sooner than boys with boys the average age to potty train a boy is around 30 months to 3 years old.
But girls tend to potty train sooner and are usually ready by 24 months.
Personally I would not even try at 18 months as that is still quite young and you'll be doing more of running the girl to the toilet yourself.
A kid is not potty trained if you have to run them to the toilet and remind them all the time.
In those cases it's really the parents that are potty trained.
A child is potty trained when they can go to the toilet on their own, wipe their butt and pull up their pants, clothes etc.
Some kids don't get potty training until after 3 years old whether a girl or boy.
In reality there's no best age but the best age to potty train your toddler whether a girl or boy is when they are ready.