Mixing two batches of chicks..when should we?

We have two batches of chicks. The first set is 5 weeks older than the 3 day olds we just got. Any ideas on how old they should be when we can introduce/mix the 2 groups. I/m kinda wondering when they will kinda be approx. the same size as each other, do we have to wait until then? we were hoping to go visit family for about a week when the new batch are about weeks old, our neighbor will be checking in on them and feeding them. I'm assuming this may still be too soon to mix the 2 groups of chicks. Any ideas???? Thanks so much.