Is 21-42 days cycle normal for a 19 year old? And its fluctuating

Depend on regularity. At age 19, menstrual periods should be fairly regular, and not vary too much from month to month. One's pediatrician or family doctor can check to see what needs to be done. Get examined, as an exam helps with diagnosis and treatment. The doctor may want to do lab tests after the exam, to check for signs of PCOS (polycystic ovaries syndrome), which is not uncommon.
Yes,but... 21 to 42 days can be "normal", but you should have a through history taken and a physical exam from your GYN.