Ob-Gyn questions? Ask a gynecologist online.
This is most likely a normal variation of your cycle. We really cannot explain why these types of variations happen. Of course, when your period is not normal, light or too early or late, and you have abnormal symptoms, there is always the chance of pregnancy. You should do a home test to be sure.
No A pregnancy test is not based on age. It is based on beta hcg which is pregnancy hormone in your blood stream. It is positive 4 days before you miss a period and have nothing to do with age. It would be positive with a tubal pregnancy most likely as well. The chances of conception past 50 are less than 1 percent. Take a test with your first morning urine.