> young people have difficulty gaining weight, no matter what they eat, until a certain age
They're not actually eating above their calorific output. This is either because they're not eating as much as they think they are, or because their calorific output happens to be high. The number of calories your body burns at rest can vary a lot, even between people with apparently similar bodies, and over time in the same body. It can even fluctuate a little in response to what you eat. So, whilst calorific intake is the most reliable single indicator of weight gain/loss, it's not necessarily useful unless you are continuously tracking it and noting the effect of small modifications. Calorie counting is psychologically difficult - albeit conceptually straightforward - which is part of the reason why many people turn to fad diets or come up with creative reasons why they don't believe in calories.
I doubt those skinny young people are rigorously counting their calories. They probably have no need or desire to. They're just eating "whatever they want", which for those people at that point in time happens to be roughly equal to their calorific output. If you took that "whatever they want" baseline and added a gallon of (whole) milk per day, you would absolutely see them start to gain weight.
> planking ... keeps my belly flat, even when I have some fat in the mid-section
Right, the shape of your belly - just like the rest of your body - is determined by the amount of muscle and the amount of fat. Decreasing fat and/or (to a degree) increasing muscle will lend your belly a flatter appearance. Planking will stress your abs and obliques, which will produce more muscle in those areas. A larger effect can be achieved by heavy compound movements, especially deadlifts, or isolation exercises like weighted situps. But if you don't lift heavy weights, which I'm guessing you don't, then planking will have a significant effect.
It's all about finding what works for you. What keeps you happy and sane in the long run. For you I suppose it's an equilibrium amount of exercise in order to keep your appearance roughly steady. For me, I love to hit targets and seeing my squat, deadlift and bench numbers go up over the year is very fulfilling. But whatever our goals our, knowing more about the subject at hand can only be a good thing.