When you live a sedentary lifestyle and don't stay physically active you increase your risk of developing diabetes, coronary heart disease, high cholesterol, getting fat and out of shape and you lower your lifespan.
If you don't stay active even into old age you risk having to use a walker, cane, wheelchair etc and you also risk having to go on oxygen and your health will go downhill fast.
I know a guy who is 90 who gets out and walks everyday as well as working his farm and he's in good shape and health.
Then again I know a 65 year old man who can barely do anything without losing his breath since he never was really active and he chose and chooses to just sit around.
Stay active and you can also likely prevent dementia by keeping your brain active and also keeping your body physically active and in great shape and you increase your chance of living a long healthy life.