I can understand the sentiment behind such a statement and suggest a possible response for someone who may receive such comments:
If someone frequently tells you that you act much older than your age, it could be a compliment, indicating that you demonstrate maturity, wisdom, or a level of responsibility beyond what is typically expected at your age. Here's a response you might consider:
"Thank you for the kind words! I think I've just had some experiences in life that have taught me valuable lessons. I believe in trying to learn and grow as much as I can, regardless of my age. It's nice to hear that my efforts to be responsible and mature are being noticed."
Remember, receiving such a comment can be positive, but it's essential not to let it define you entirely. Embrace your unique qualities, interests, and age-appropriate experiences as well. Strive for personal growth, but also allow yourself to enjoy the journey of life at your current stage.