Yes, it's fair for your children to recieve different allowances, just explain that as they get older, it can grow-maybe by offering a dollar/year old/week.
For instance your thirteen year old could recieve 13 dollars a week for her chores, and next year bump them each up a dollar.
Different ages can do different varieties of chores, the oldest obviously being able to complete more, and also at an older age, probably has more expensive taste.