Hi!
There are quite a few answers to this question, and some of them depend on what is available in your area. Some stores/brands offer a "grower" feed that you feed after about 8 weeks or so. Others offer a starter/grower all in one that is meant to be fed up until point of lay. If you are raising several generations together - or several different breeds of birds - you might consider a more general product such as Flock Raiser, which is what i use for all chickens at all ages.
If you have a flock of laying hens, you will want to switch to layer pellets around twenty weeks, or when the first ones start laying eggs. (Unless you're doing the flock raiser thing like i am.)
The grit question has a lot to do with their environment. I don't give my chickens grit, but they pick up lots of rocks from the yard, and they free range, so they have lots of opportunity. If you have any question about whether your chickens are getting enough grit, then you should offer them grit, separately from the feed, so they can get it if they want it.
I hope that helps!