There are two only tangentially related issues here. One is, given you're already overweight or obese, can exercise help you change that? The answer seems to be

There are two only tangentially related issues here. One is, given you're already overweight or obese, can exercise help you change that? The answer seems to be yes, it definitely can, but it seems empirically that it very rarely works over any appreciable long-term, other than in people who find they enjoy it sufficiently to become at least recreationally serious endurance athletes or at least very active people for the rest of their lives. If your only motivation for exercising at all is to lose weight, it very likely isn't going to work over any meaningfully long span of time. But the other issue is, given you're a normal-sized, metabolically healthy 10 year-old, will developing a lifelong habit of regular exercise and a generally active lifestyle keep you from ever getting fat in the first place? I don't know that we have any kind of long-horizon, longitudinal research to demonstrate one way or another empirically, but mechanistically, it sure seems like that should be the case, and you can look at manual laborers in developing countries, the Amish, any serious lifelong athletes in sports where being maximally large is not advantageous on its own, as existence proofs that this should help. I think the comment you're responding to was really asking about the latter scenario. It's not, okay, you spent the last 20 years gaining 30kg, well how active are you now? The question is how active were you over the entire course of the last 20 years? Because it's likely that if you were sufficiently active, you wouldn't need to lose weight at all, and the question of exercising for losing weight would be irrelevant.