My mummy hen starts encouraging them to eat greens around day three or four, so if I'm raising them, that's when I start giving them greens.
I like to give them a little at a time, chopped fairly small the first few days. Uneaten leaves can go moldy and harm the little guys, so I like to clean out any uneaten greens once a day.
Personally, I don't like to feed them anything I like to eat. What they eat at an early age seems to become their favourite, so they get a nutritious variety of weeds. It's your choice which plants you choose.
You can feed them plants at a couple days like already stated, but it's a good idea to give them grit if they don't have access to any dirt. When chicks are with mama, they are usually outside and can find their own grit to break down the plants but in a brooder, they don't have that option. Only costs a couple bucks at the feed store and they only need a little bit. Once they are outside, you don't need to provide this anymore.
I feed mine buckets of garden soil at about day 3. This seems to encourage strong foraging behavior, and contains grit and bugs. Greens in small amounts usually at about five days or so, chopped or torn up into small bits.
I just had the craziest dream. This tiny ad was in it.
Heat your home with the twigs that naturally fall of the trees in your yard