I have a dime size knot on the top of my head. I've had it since 2000. Should I worry?

Pilar Cyst. 9 times out of 10, what you are describing will be a type of blocked oil gland called an inclusion cyst or pilar cyst. These can be easily removed under local anesthesia if they interfere with hair care. There are other rare growths that can occur on the scalp as well, so i advise you to see your primary care doctor or a dermatologist for a formal evaluation.
Have to see it. 12 years a long time and less likely to be serious. We can have moles or seborrhea changes that are nothing to worry about. However solar keratosis or even some basal cancers can be very slow growing. If the lesion is irregular in shape, unusual colored, bleeds, or has been growing more rapidly it should be looked at.
KNOT ON SCALP. Sounds like a cyst. See your dermatologist. Does not sound worrisome.