I know there is a lot of snake oil out there, but is there anything that can truly reverse the hair thinning and/or graying process from within? through nutrition or other means?

Hair thinning: You're very smart to be skeptical about untested treatments advertized online and elsewhere. There is no product that prevents or treats hair graying with age. (Not counting hair dying, of course.) There are prescription drugs (finasteride, minocycline) that can induce hair growth and perhaps treat thinning -- not highly effective, but sometimes helpful. Discuss with your doctor.