When to mix? (juveniles and littles)

[EurekaPaprika]

We bought chicks from IFA this year and they are now about 6-7 weeks old (5 chicks). I've put them outside in our coop and they're doing just dandy.
My niece volunteered me to take the chicks that her school class hatched, and they are now about 3 weeks old (3 chicks).
I've never had two broods of chicks to combine, at what age will that go most smoothly? Any tips on how to do it?

 

[SueT]

Temporarily divide them so they can see but not touch each other-- that is step one.
Here is an article on adding newcomers, the first half deals with quarantine, the second has details on how to combine them:
https://www.backyardchickens.com/articles/adding-to-your-flock.47756/
There are a lot of forums about integration chock full of useful advice, experiences etc...
Good luck!