I've had gray hairs pop up since I was about 16, only creeping up more often now and more disperse (32 right now), though that was the same for my grandpa and dad (full head of gray hair), so it didn't surprise me. Still have enough dark hair to counteract it and not get the salt and pepper vibe going on.
With kids and responsibility / tolerance, after I had my first, I didn't think I could handle any more (she was particularly needy / demanding as a toddler, much better now as a 5yo). Luckily my second was an easy newborn to help ease the transition, but became inquisitive (read destructive) when she hit 2 years, so that became a combustive nightmare when they were together.
Now with a third and the two older ones becoming more physically and verbally combative every day, I don't know how its holding together mentally for me and my wife, but somehow we manage day by day. The point is I realize now how good I had it with one kid, even two, and how most of the pressure then was self induced.
Learning to let go on things that don't really matter is essential, so you don't realize how much you can handle until you are forced to do so, and then loving the small rewards and awesome parenting moments amongst the chaos.