When selecting a birthday gift for a mature girl, consider her tastes and preferences. You could opt for a personalized gift, such as a custom-made piece of jewelry or a monogrammed leather journal. Alternatively, a spa day or wellness retreat can offer her a chance to relax and rejuvenate. If she enjoys the arts, tickets to a theater performance or an art class might be appreciated. Remember, the best gift is one that reflects her personality and interests.