The idiomatic expression "to go gray" means to become older, especially when referring to one's hair turning from its natural color (usually black or brown) into a greyish hue due to aging. Full definition
In between coloring, I can see I have my own strands appearing in different places and I'm not ready to let my own hair go gray yet. (finewhateverblog.com)
It was so intense that I started going gray at the tender age of twenty - six, and I had been plagued with insomnia for over 18 months! (parsleyhealth.com)