Answer:
straight hair cannot become curly on its own, that is the way it grows out of your head, so if you want curly hair, you can get a perm, however, go to a salon to have this done, if you do it on your own you may wind up with a frizzy mess.
From the author of the original question: Perhaps more information is needed to clarify the question. At 32 years of age, my hair has gradually become curly, over the last 6 months or so, after a lifetime of being bone straight. Is there, perhaps, an undiagnosed medical condition which could cause this to occur? I am stumped as to why it has changed. Any help, or even guesses at this point, would be appreciated.
I also had this happen to me. My hair was bone straight until I was about 30. It has since become more and more curly. To the point that it is now difficult to straighten. My only explanation as to why this occurred is hormones. About the time my hair started to curl, I gave birth to my daughter. I think the hormones involved in pregnancy or breastfeeding caused the hair follicle to change. My hairdresser is of the same opinion as the same thing happened after her first pregnancy. Hope this helps!