Gaining muscle mass will often lead to a quick onset of obesity once you stop lifting.
What I mean is, the fact that you have muscle mass and stop lifting is not what's going to lead to "a quick onset of obesity." What leads to weight gain is still eating like you're lifting when you're not.
If you adjust your caloric intake to account for your new level of caloric expenditure, you will not gain weight, although you will likely see unfavorable body composition changes.
Yeah, that's what I meant. So it is not nonsense.
Gaining a lot of muscle and putting a lot of effort into gaining the muscle (bulking + workout) is where the troubles maybe appear.
Also, the metabolism of a 20yo is a completely different story frok the metabolism of a 40yo.
So to sum up: working out and staying in shape == good. Don't worry about gaining muscle. That's just nonsense
People that lift often eat more. But mostly protein, and stimulating muscle every day makes sure it will get developed and consume more calories to maintain itself.
If you stop lifting altogether, not only muscle mass stops growing but also starts to shrink. If you eat the same, you will gain weight faster and with less food than before.