My mom began potty training me at 18 months of age and had me out of diapers before 2 years of age.
But some kids today are still not potty trained and are still in diapers full time as old as age 4 years.
Potty training should at least begin at 18 months of age to give it a try and then if they don't want to potty train or don't seem ready to potty train at that age then you can try waiting until 2 years old to try again.
Most kids should be fully potty trained at least during the daytime by age 4 at the latest and if they still aren't potty trained and out of diapers during the day at age 4 then you really need to start forcing the issue because a kid won't be accepted into Kindergarten wearing diapers unless there's an actual medical need or disability that requires the diapers.