Most people I know from childhood have kept the same kind of BMI/body shape they had as a child/teenager. For example, those who were really skinny as teenagers are still skinny as adults. Those who were overweight as children are now overweight adults, etc. Obviously excluding any specific causes for changes in weight such as health conditions, pregnancies, etc.
I was slim my entire life until my early 20s, and during my 20s I've put on a lot of weight and weight from a size 10/12 at 20 to now a size 18 at 28. I've not had any pregnancies or have any known health conditions that impact my weight.
I'm trying to figure out where it went wrong... I don't know anyone else who has gained a lot of weight in their 20s. I'm wanting to lose weight now and I'm thinking of trying to go back to what I used to eat when I was 20. It wasn't particularly healthy, but it was 'normal' food and small portion sizes.