Hoping to determine approximate age

[Mystery-chicken]

I have 4 Orpington mix chicks. We got them 3 weeks ago and could immediately tell they were not all hatched at the same time. The white and gray ones specifically already were mostly feathered. The two brown ones were barely feathered, but still obviously different ages.
I have a lone 12 week old pullet as well, and would really like to get them introduced (properly) once they’re all feathered out and a bit bigger, but I for sure want to ensure they’re all over 6 weeks.
Any help is appreciated!

 

[rosemarythyme]

#2 is probably about 3 weeks, #4 is 3-4 weeks. The other two are older but it gets harder to tell the more feathered they are, 6, 7 weeks +?

 

[rural mouse]

And 4 looks like a cockerel....


As far as introduction, early is good. 1: LOTS of space, what you think is roomy, double that, then double it again.
2. Line of sight blockers without places to get cornered.
3. Multiple feed and water stations, also where getting cornered isn't possible.