At a couple weeks old, ours do fine outside during late March/early April (Texas, warm climate).
They will need chick grit if they are in the brooder most/all the time, but we don't feed grit once they are free to roam outside, they find their own, but they have a lot of space to range over.
Baby chicks like "grown up food" and they will begin foraging immediately if you let them, and love bugs and greens, etc. You can just collect some kitchen scraps, weeds, bugs etc. (a black soldier fly or red worm composting bin is great for this) and toss them in the brooder. They will fight over it. This also has the benefit of introducing it slowly if they have been on all commercial feed, since their systems are pretty delicate and a sudden change can lead to "pasting up" or sudden random death. They love fruit and starchy vegetables such as squash. Mulberry
trees are a winner if you want to grow feed. Chayote is also particularly beloved by my chickens, and is a perennial squash (delicious for humans, too). Basically anything you grow an excess of or that drops fruit. If you use a deep litter over a dirt floor, they will hunt through it for bugs and such once the litter matures. But for babies that are not used to a diverse diet, slow introduction is usually best. If they have been foraging with mama hen from the beginning, they do fine on a diverse diet.
After a week or so, we started letting them out on "supervised" excursions of half an hour, a couple hours, a few hours etc. until they got a little bigger, always making sure they could return to the brooder if they got chilled. Once they were feathered, the brooder went and we locked them up at night and let them out in the morning to free range. Many are lost to predators during that stage. We were okay with that because we want to select for survivors. Now we just leave a small door open so they can come and go as they please night or day. Occasionally we lose one to predators--our predator pressure is actually very high here, to the point that many people cannot keep chickens at all--but most of the stronger and cannier ones that are left--the survivors--do just fine. We also have a rather high ratio of roosters to hens (two or three to ten, usually), which is kind of hard on the hens but helps with predator issues--we also don't like to cull all our roosters and then have the last one get eaten by some critter and have to go find a new rooster, so we keep two or three. We also try to select for "dancing" roosters that will do a mating dance rather than brutalizing the hens. We haven't had the best luck so far. But this would make our high rooster ratio much more manageable. The other issue is feed--since ours forage for theirs, we can afford to keep roosters. If you are buying it or growing/gathering it yourself, you probably don't want to feed extra unproductive roosters.
It depends on the breed, as well--some are certainly hardier than others.
Games are the best survivors and foragers and the best (in my
experience) at hatching chicks--they will just appear one day out of the brush with a brood of them.
We don't feed supplemental feed (well, sometimes my dad gives them
deer corn because they start going after the dog/cat food, but even without the corn or pet food they do fine and lay regularly). If you have enough space, they can find their own food, although they will welcome scraps. If not, you will probably need to grow or collect food specifically for them. I have yet to encounter a fruit they do not want to eat, so if you have fruit trees whose production you can't keep up with, you are set.
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