Why are you stating a hypothesis as if it was fact? I'm 30 and started learning Spanish (1,000 hours so far) and think I'll be near-native level in another 1,000 hours and maybe actually native in 5,000 total hours which I might get to by the time I'm 40. As we age, we simply have other commitments that we cannot devote this much time to language learning. Kids "easily" learn language because they can easily put 1,000 hours of exercise each year for 5 years.