When I was young I had blonde hair, as I pass 40 my hair is getting darker when most are turning gray why?

I don't have an exact answer why but a lot of things change with age especially hair. Things like color, texture, porosity. Certain medications can change your hair, or if you have thyroid problems. Ive noticed with my clients that have had chemotherapy, their hair falls out (not always though). But then it come back darker, curlier, or even thicker.