I think the changes happens for a few reasons 1. Age - assuming we study a give style our entire life we should be more physically fit and healthy in that 18-32 age area. When we were younger we may not have had the strength or when we were older the speed, flexibility, reflexes etc. 2. Mental growth - because the more we study the more we understand what we are doing and how to do it. Let me be clear I, nit taking about Kia or th force. However mover application and body alignment can allow a older person to preform better because they are maximizing all their ability. This is also a reason I don't allow black belts under the age of 15 I know now that I'll never do what I did 15 years ago but that don't make me a worse martial artist. No matter how hard I train things get sore quicker and pain lasts longer lol. However, I find myself moving with greater efficiency and I have became much more precise and l as wreckers with age Long story short if your no changing or adapting your not growing. My karate is not your karate althoughwe train the sa,e style from the same teacher in the same dojo. We must adapt to fit our body, age, ability, handicap, lifestyle, environment, laws, etc