My Bangs never grow longer than 2 1/2 inches since I was 10 years old. Now I am 40, why is that the case.

You have short bangs and have had them many, many years (over 30 years). I suspect that your frontal 1/2 inch of your hair line has a short anagen cycle reflecting the fact that the bangs do not grow to length beyond just a few inches. This is your genetics and there is nothing that can be done for it