Its based on our age how quicklyy we lose or gain weight? And if yes, why?

You lose or gain weight based on how much or how many calories you eat and NOT What You Eat.

Lets say your metabolism burns 2000 calories per day or your body only needs 2000 calories per day to maintain your current weight but you constantly eat 2500 or 500 extra calories everyday then you're going to gain weight because your body takes those extra calories and stores them as fat and it DOES NOT matter where those extra calories you eat comes from.

So if you eat too much or too many calories (of anything whether its healthy or unhealthy) you're going to gain weight.

This is how you lose weight

Again, lets say your metabolism burns 2000 calories per day but you constantly eat 1500 calories per day then You're going to lose weight because you're not eating enough calories to maintain your weight.

Whenever you don't eat enough calories (regardless of what you eat) your body will burn off your bodyfat for energy (causing you to lose weight) to make up for the lack of energy/calories you're not eating.

See how many calories you need to eat to lose weight based on your weight, age, height & activity level.

Decrease or weight gain depends on the health status, diet and physical effort .