Since it's a topic of the week I'll pipe in here. More details on
my early integration approach in the article here and
2024's integration thread here.
How old should chicks be when they are integrated into an adult flock?
I believe in early integration, so "as early as possible." I aim for partial integration (chicks spend days with adults) at around 2 weeks old and full integration* (chicks spend all day and night with the adults) at around 1 month old.
*I call it full integration but note that the chicks won't become part of the flock and pecking order until they near point of lay. They don't really start mingling until then.
Are they too small to be integrated? Do they need to be a similar size to the older chickens?
Smaller is better - they're faster and more agile, able to utilize their small size to hide or pass through spots that adults can't fit (hence the chick-sized openings), and the adults don't view them as a threat as they would an older bird.
What is the best way to introduce chicks to the rest of the flock?
Brooding outdoors. If you brood chicks in view of the adults you can technically start letting them out with supervision once the adults lose interest in them and they're steady on their feet. For me that's usually around 10 days old.
By roughly 14 days old I teach them to utilize panic/chick openings to run to for safety when needed. When they show me they know how to use them, they're ready to have full run and coop access with the adults all day.
At around 1 month I close off the brooder and the chicks are integrated into the coop.
What / how do you feed a flock of mixed ages after integration
All birds eat whatever the youngest birds eat. Oyster shell or crushed egg shell in a container provides calcium for laying hens, and I keep both grower and layer sized grit available full time.