Flowergirls and Pageboys are usually nieces and nephews or young brothers and sisters and are generally no younger than about 5 nor older than 9 or 10. Children under 5 are unlikely to understand much of what's going on and may decide to start playing hide and seek just as the vows are about to be exchanged.
Flower girls walk in front of the bride carrying posies of flowers or bunches of thornless roses, which they can pass out to the guests as they go. They can also strew rose or other flower petals before the bride as she walks down the aisle.
Pageboys traditionally carry the brides train if she is wearing a dress with a long one. They can also be used to carry the rings on a cushion to the couple.