I would mix it up! Shoot a combo of mature and young does. They (biologist) say that the older does are the keepers of the herd, being that they can protect little ones from danger (yearling bucks too) such as cars, yotes ect ect.... and the abandantment rate is lower with older does. But on the other hand you don't want just older ones breeding exclusivley if they are past there prime. I would combo it up and even take out a few of each and also some 2+ year old basket racks too.