You will get fatter unless you eat less and less for the rest of your life, probably starting around 32
The healthier you eat, the more difficult it will be to become and stay overweight.
A McDonalds hamburger with a medium fries and medium drink is 1,200 calories. This is almost trivial for most people to eat.
Conversely, 1 lb of lean ground beef, 2 cups of rice (when dry), 2cups of vegetables with a dessert of two cups of fruit is also 1,200 calories, yet represents almost 3× the amount of food when fully prepared.
Eat healthy raw foods, and viciously limit your intake of highly processed foods, and you will likely remain decently slim without even trying.