Once upon a time, a bride had bridesmaids to serve as physical protection from ne'er-do-wells. Today, they offer the bride emotional support throughout the wedding planning process and stand by her side on the big day. No matter what the age of the bridesmaid is, she's expected to take on certain tasks associated with the wedding. She'll have to buy a bridesmaid dress, help throw or plan wedding showers and the bachelorette party and buy a gift for you and your soon-to-be hubby.
One thing to take into consideration if your bridesmaids are a little older is to pick age-appropriate clothing. That cute, kinda low-cut dress with ruffles and a sash that's perfect for your 23-year-old sister might not look so great on your 33-year-old cousin. Think about finding one dress that's appropriate for all ages, or choose a color and let your bridesmaids pick their own outfits. Even if everyone wears a different dress, they can still look coordinated.
Also keep in mind that older bridesmaids may be beyond the clubbing phase, so when you're throwing out ideas for the bachelorette party, make sure everyone's comfortable with the plan. A bridesmaid who's over 30 may look forward to a night of dancing, but might not be so keen on wearing novelty necklaces or hats that identify her as being part of a bachelorette party. She'll want you to have a great night, of course, but making her feel comfortable and part of the group will help her, you and everyone else relax and focus on having fun. If you still want the crazy party, consider giving her a pass if she's going to feel like the odd woman out. Instead, maybe consider throwing an additional, more low-key celebration like a day at the spa that all your bridesmaids, regardless of age, will enjoy.
Ultimately, the role of bridesmaid is all about being a true friend or family member, and age has nothing to do with it.