Comment by nradov

There's a huge variation between individuals based on size, sex, and activity level. As a large, fairly active man I have to consume around 3100 kcal per day just to maintain body weight. But a small, sedentary woman might gain weight with even half that consumption.

All else being equal, resting metabolic rate doesn't tend to slow down much as we age. The notion that people in their 20s can eat as they want without gaining weight is mostly a myth. It's more likely that you don't accurately remember what you consumed and your activity level in your early 20s.