I was told by a neurosurgeon that my neck bone shows aging. I am 21 years of age. What does this mean? Can I reverse it?

Aging bones. When bones show "aging" it means that the effect of gradual wear and tear that we all experience is starting to show up as mild degenerative arthritis in your spine or limbs. While this is unusual at age 21, it certainly can happen, particularly if you have had a major fall, car accident, etc. The only thing you can do that will help is regular exercise with stretching. Check another xray in 5 yrs.