That's why kids who grow up eating unhealthy can look healthy for a while, but eventually they look like their parents. And once the body is out of buffer, any extra calory goes straight into storage
Basal metabolic rate does go down with age, but basal metabolic rate and total energy expenditure are mostly stable between 20-60. The basal metabolic rate of a 60 year old is about 120 calories less than that of a 20 year old. And actual maintenance calories are 200-300 lower for people with low to moderate exercise.
What happened is that the kid is either eating more, or is exercising less.
A normal 20 year old who isn’t an exercising a significant amount can’t regularly eat 3600 calories and maintain a healthy weight. That is to say, the average 20 year old with a healthy BMI who exercises 1-3 times per week will gain somewhere around 2 lbs per week eating 3600 calorie per day. They’ll gain less over time if they don’t start eating more as they get closer to 350lbs because their maintenance weight will go up.
And despite individual variation in BMR, I haven’t heard any evidence that some people have the ability to significantly ramp up their metabolism in response to consuming more calories.