I went gray in my early 30s, tried dying it for a couple of years then gave up and started shaving my head. Twenty years later I came to grips with the fact that gray/silver hair actually looks pretty cool and started growing my hair out again ... only to discover I'd gone bald in the meantime. Oh well, hair today, gone tomorrow.
Some guys go grey and have the old guy look from early. I remember as a kid my dad being grey haired and me seeing him as old, but he was 35 and completely grey.
Now I'm in my early 40s and still have jet black hair, and look 20