why would you want gray hair? and there really is no way of knowing. some people start to get gray hairs as early as their 20s, others reach 60 and have no gray. It really depends on the person. I think the average age to start graying is mid to late 30s and then many people are fully gray in their 40s to early 50s but it really varies. I work with a lot of people in their 40s and 50s, most of the men that age are salt and pepper or fully gray.
There is no way to make it faster, no.
Also if you are a natural light blond you may not get so gray just fade to white eventually.
There's no way you can possibly tell. Its a mix of your genetics as well as stress level. I don't know why you would want to make it faster. Even if its for a fashion trend, I'm sure you would regret it once you got them.
you're only 19,if you have healthy genes and you aren't stressed-you'll have pretty much your entire life so far of blond hair in front of you. stress can make your hair grey-a friend of mine had some gray hairs in elementary school,cause of stress.