When to move chicks from the brooder to coop?

I moved my chicks outside at 3.5 weeks when it was that temp outside with no extra heat.
I'd put the younger chicks out with them at the same time. You could provide extra heat if your more comfortable with that, but I think they'll be fine.
Have they gone outside in the day time before?

 

My chicks are 3 weeks and have been outside with supplemental heat for 4 days. Nighttime temps are in the mid 20's and they've been just fine. They all have quite a bit of their feathers in too.

 

Jazzy, you need to see this! https://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update I think this would enable you to move both sets of chicks to the coop right away, avoiding the reintegration problem later on.

As far as I can see, this method of heating chicks simply has no downside whatsoever.