Weddings typically have a wide variety of participants. A fully decked-out ceremony might have 20 people or more on their feet that day: the happy couple (of course), an officiate, a best man, a maid of honor, attendants serving as bridesmaids and groomsmen, a ring bearer and a flower girl. You might also have readers and musicians who are part of the service.
If you have someone you'd like to be a part of your big day but you're afraid that she's too young to be a bridesmaid and too old to be a flower girl, you might consider asking her to be a junior bridesmaid. Junior bridesmaids are usually 9 to 14 years old and have fewer responsibilities than full-fledged attendants. They wear dresses similar in style to the bridesmaids, and they stand with the other members of your wedding party. But with a junior bridesmaid, you'll have more flexibility with what events she attends. You're free to have her there only on the day of the ceremony, or you can invite her to other bridal events that you think she would enjoy. For example, you might not invite her to the bachelorette party, but you may ask her to attend the bridal shower and take her with you when you go wedding dress shopping.
Ultimately, there's no special formula for determining how old your bridesmaids should be. You want to select people who you love and are comfortable with to serve as attendants for the ceremony. If you have a close friend or family member who's 14 but looks older, feel free to make her a full bridesmaid -- as long as you and the young woman are comfortable with her serving in that role. Chances are, on the big day, your guests won't be worried about the age of your attendants. They'll be focused on you, your new hubby and, of course, your dress!