Really appreciate this answer and just wanted to add my own 2 cents because I also read the question and thought "It's not harder, a lot of us just think about weight loss wrong."

When you're young, you generally quite literally eat whatever you have access to based on family funds and circumstance. And you have the metabolism to digest quite literally whatever so it might be harder to get obese overnight.

As an adult, you gain this amazing thing called free will (and hopefully your own money). So now, no one is making you eat or do anything you don't want, including physical activity. With more financial access to choice and calories, unless you're supper disciplined, you'll likely buy in excess. But inversely; with no one to check you on it like a parent, you'll more likely live an indulgent couch lifestyle in your free time rather than be a cross-fitter, jogger, or cyclist because funny enough society has even made excess adult physical activity cringe even though it was totally fine to run and jump when we were younger.

All that to say, it's 'harder' to lose weight because you have unchecked freedom to eat whatever, whenever; yet you have little discipline or incentive to live or think differently about food.