Almost 2 months ago I purchased a 4 month old male redfoot. We also have a 4yr old female rf. How old does the lil one have to be to live with the female?
I would say when they are close to the same size. There will be a point when the older ones growth will slow and the little one will catch up in size, but I wouldn't until then.
If you are going to put them together, I agree that you should wait for the little one to catch up with the size of the older one. BUT. It is not recommended to have them together in pairs anyway, unless you want to try and breed them.