As they age they should have larger grit. Unless you've only got beach sand I imagine they'll find all they need for grit. Your birds are at the age where they really start packing in the feed. One of many reasons why I butcher cockerels at 12-14 weeks. Free range really helps in feed cost. Depending on quality of forage area and size will determine how much it offsets the purchased feed intake. Sure it did wonders the first week but soon they will deplete that natural resource if not a large enough area or quality. No idea where you live but if you've neighbors close there will come a time when you can't let them free range without a fence of some sort. As they pick clean the forage closest to pen they continually start to range farther out. Just a heads up on natural chicken behavior so you are prepared.
The real gig with grit is it's the chickens teeth. The can eat anything without it they simply can't grind it up as much to extract all the nutrients before it exits the body. What that boils down to is they will eat more feed. Grit is great and completely natural for chickens to get on their own from the ground. Where supplying grit comes into play is for confined birds without access to soil. Winter in the frozen north is what I deal with. Pre winter we gather small stone from the driveway and put in a bucket. If there are no thaws over winter then I toss a handful now and then on the ground. Is it essential I do that? No. It keeps the feed cost down and allows them to get most nutrients from food scraps they are fed.