I haven't given them grit, except from clods of dirt/grass that they peck at. Do I need to buy something, or can I give them something from the yard? I have a lot of wood ash, should I not use it at all?I would not use straight ash. Best thing would be to supply regular dirt or peat moss.
A week old should be fine.
Are you offering grit in a container for them to get as needed? They will need grit available if eating anything other than chick starter.
Have you ever had a chick bring you a large bug, pick it up and drop it looking up at you...then you realize they can't eat it in one bite and they're "asking" you to cut it up? I did, cringing, she gobbled it up and ran off for more.