One of the most productive engineers I knew (who I’m guessing will read this) was in his 60s when I met him. He’s since “retired” and helped a company obtain a $1B valuation. Similarly, my 85 year old grandfather still doesn’t own a wheelbarrow and will move stone, dirt, etc every summer. His motto is “keep moving”, I don’t think there’s really anything else to it. Yes you will slow down, but stay engaged and motivat…
Keep learning, keep moving your body. When you're 15 this is easy, you don't really have a choice. When you're 40+ it starts to become more of a deliberate act.