There is nothing special about the mid-20s that makes you less hungry, biologically. It's just cultural: in our culture, it's typical to be settled into a way of life by your 20s or 30s. That doesn't put up a real roadblock unless you let it. The research you're talking about wouldn't, I'm guessing, contradict this.
Well I was more focusing on the effect than the cause since this is a cause in the current context (i.e. lack of 'hunger').
But yeah, most of it IS probably driven by culture. That said, I'm sure there are some biological intricacies too. Something like x years after puberty your brain starts to want more of the same things - something that would definitely help in establishing a stable lifestyle and child rearing.
