Your health can drastically change in a matter of 1-3 years, young or old, especially if a physician is not evaluating you. With that said, there is a way you can do a very basic check up on yourself without having insurance - this does not take the place of a full physical, but it is a starting point.
You can pay a cash price for a doctor visit, which is less than what you'd pay if you were paying for insurance every single month.
You can do your own blood work through a place like Any Lab Test Now, which allows the public to get their own blood work done, and if you notice anything abnormal, go see a doctor. The two main tests to obtain, at a minimum, would be CBC (Complete Blood Count) and Comprehensive Metabolic Panel (CMP).
Know what chronic health problems run in your family and live your life in such a way as to prevent those things from happening.
Ex. if it is hypertension, buy your own blood pressure cuff and check it at least once per week, while in a relaxed state, and uncross your legs.
Ex. If it is high cholesterol, get a full 'lipid' panel done with the blood work.
Lastly, even if you are not paying for insurance, you can create your own mini insurance policy for yourself. In other words, save money every month that would go towards medical bills should you ever incur any.