Flower girls are usually between 3 and 7 years old.. Since children tend to be shy in front of a lot of people, it is usually best to leave them sitting with their parents. Young flower girls more often than not end up being dragged down the aisle with tears streaming down their faces. But it really depends on the
child, so discuss it with her mother first. 
An older child between 8-12 (average) can have a  more grown-up role in the ceremony by making her a junior bridesmaid. This acts as a way to include her in your wedding and make her feel like one of the "older girls."
It is very common to have more than one flower girl.. Usually, they walk down the aisle side-by-side, preceding the bride. They can toss petals from a basket, or simply hold a basket, bouquet or flowers. 
Flower girls' dresses should be the same color as the bridesmaids' dresses or all white. Junior bridesmaids can wear the same dress as your bridesmaids if appropriate, or  a younger, less-revealing version of the dress.

 

 "We had four flower girls in our wedding. Age 7, 5, 4, 3 and a ring bearer that was 1 1/2.  The parents of the children being seated in the front already,  We found their weakness, for the girls it was Beanie Babies, for the boy, fire trucks.  They knew before heading down the isle there were presents or "rewards" waiting in the front, but they had to get their first. (I am sure their parents spoke to them as well).  I must say they were marvelous!  I was surprised, as I expected some shyness and reluctance.  Especially when the children are from 4 different families.  

We had them carry small bouquets of fresh flowers and each had a silk flower crown which was their to keep as well."

---Danbury, Connecticut