At what age do periods become regular?

Varies. This is a variable answer.They can be regular from the beginning, take several years or never short of hormonal therapy.But other than uncertainty, should not really be a problem.
Menstrual cycle. In the first two years after starting your period, your cycle can be irregular, and that is normal. Although having a period frequently, like every two weeks is a reason to see your doctor. A period becomes regular when a woman/girl begins to ovulate regularly.