Islanders get fat more easily than the rest of the world because they are descended from the people who survived 1000+ mile voyages on a flimsy canoe.
Only the fattest women were sent because they could store enough food to survive the trip. By the time they got to the next island (if they found one) they would be skinny. So you could not send skinny people.
So the current population has been artificially selected (by design) to survive starvation by storing fat.
If you've ever been to Hawaii you will see the natives are all fat from American food. They no longer eat native food any more.
And yes, when you guys get older, your metabolism will slow down. You won't be able to exercise as much because you won't have the energy, and your body will hurt more easily and take longer to recover. And you won't have the time with work/school/kids.
You will need to cut down on your calories and make a deliberate effort to exercise consistently. Avoid fatty foods, trans fats (hydrogenated oil), alcohol, salt and sugar.
I neglected my body until my late 30s, and you probably will too, but it's never too early to start living healthy. I am much healthier now than I was five years ago.