In all the years I have raised rabbits, I have never seen two different age litters carried at the same time.
........I did have an unexplained litter once, from a doe I was going to cull, she had been placed in a separate area after weaning, awaiting dispatch, -- but, i wanted to let her gain a pound or so --so she was not butchered right away.- somehow- she had a litter about 2 months after moving her-- there were no buck rabbits in that area...