When can I mix my betta babies?

[On_a_dishy]

I have 4 x 5 week old betta babies in a Fluval 34 litre tank.
I also have 5 x 3 week old betta babies in an unfiltered tank (cleaned daily with twice daily water changes).
The tanks have always been next to each other and the fish sometimes swim up to look at each other. In the photos the older babies are on the left. They are all healthy and active and I've started weaning the older ones onto all sorts of food although I still give them baby brine shrimp as a treat.
Can I move the 3 week olds in with the 5 week olds?

 

[itiwhetu]

I would say so. At some stage you are going to need some way of separating the males from each other.