How old should children be ? It depends on the child's personality is the key. Are they shy? Have they ever been in a wedding before? How do they feel having many "eyes" looking at them as they walk down the isle during the wedding procession.
Flower girls...I would think carefully before you expose a young child to this task. Are they outgoing? Have they been in a wedding? If they have this is a "plus" in your favor, because young children can be unpredictable and "freak out".
Purchasing a dress for a Junior Bridesmaid or your Flower Girls cannot be done way in advance. Why? Because they are still growing like weeds. In view of these facts, you still need to think 12 weeks ahead of the wedding date..that is the day they are wearing the dress, which is the day of the wedding!
In placing your order 12 weeks ahead, consider the child's size at the time of the order, AND the pattern of their growth pattern within that year. Ordering their dress 2 sizes larger than their regular size is a safe guide to follow. Always bear in mind there will be the cost of alterations which have to be factored in!