Well, I'm a brunette who is approaching 40 and my hair is becoming increasingly gray.
What I've decided to do is get all of my hair lightened one shade lighter
so that when the gray comes in, it's less noticeable and not such a contrast against the darker hairs as the color fades. Those formerly gray strands simply look highlighted over time.
And my darker roots are NOT that much of an alarm and shock to the eyes of others when they grow in since I've only gone one shade up. Pretty clever.
I think it's a lot less hassle and work to go a shade lighter all over than to attempt going darker and dealing with gray that fades and refuses to stay as dark as the rest of my hair.