Many modern lifestyle diseases such as heart disease, diabetes or dementia, do not suddenly occur in old age. They are formed in your 20s, 30s, 40s and 50s. It is only that the symptoms become significant in old age.
To help prevent them, you need to focus on good sleep, healthy eating (eating less processed food, eating more diverse food, drinking less alcohol...), regular exercise and participating in good social relationships throughout your adult life. What you do in your earlier life sows the seeds for your later life, so don't wait for tomorrow to make changes.