when do you start feeding grit ?

At 14 weeks your hens will be just about adult size, and you'll find over the next year or so they will "beef up" and get broader, bit not necessarily bigger (longer/taller)

There are two kinds of grit... hard or flint grit - this should be available all hte time, as this is what the hens eat and 'store" in their gizzard to help grind up the food they eat.

Then there is the oystershell.  This is contains soluable calcium and helps the birds form good egg shells.  It can be argued that in modern times with scientifically monitored compound feeds auxilliary supplementation of oystershell shouldn't be needed... but better be safe than sorry

You will need to buy some flint grit too   If your hens free range, they will naturally pick up some small stones etc from foraging inthe grass/ground.  but if they have a special pot of hard grit, it will make life easier for them