It’s really valuable for children to learn in mixed age groups. Age mixing allows younger children to engage in and learn from activities that they could not do alone or just with playmates of the same age. They observe and emulate models of activities more advanced than their own; and receive emotional support and care beyond that which age-mates could provide.
Age mixing allows older children to develop their capacities to nurture and lead and allows older children to expand their understanding through teaching. Studies have shown that age-mixed play and learning is generally less competitive and more creative.
In choosing your learning aims you can try to be aware of the optimal age range that might be suitable for your particular learning experience. It’s not necessary to expect every child to become competent in every learning aim by the end of the experience. The outcomes will depend greatly on age and individual differences. Exactly the same principles apply for mixed ability groups.