I remember hearing a guy say something along the lines of: You do all these things to extend your life, but you're not extending your 20's and 30's. You're extending your 70's and 80's.
Depends on what you mean. A healthy 40 year old has more stamina and well-being than an obese 30 year old. So at least in one way, you actually are extending your 20s, and your 30s, and your 40s, and your...
Staying healthy is not just about living "just a few more years". It's about quality of life. Being unhealthy at 70 is painful and expensive.
Similarly, being unhealthy at 20 is a waste of time. You can eat cheetos at any age.
A similar quote that I prefer is (something like) "stopping smoking doesn't make you live longer, it only feels longer".