An interesting perspective is putting in the years of learning. Say a kid realistically starts learning a language at age 4. Add 5 years and you end up with a 9

An interesting perspective is putting in the years of learning. Say a kid realistically starts learning a language at age 4. Add 5 years and you end up with a 9 year kid. 9 year olds are not thaaat good at language. Compare this with an adult that's been learning a completely new language for 5 years. As in seriously learning, dedicating at least one hour per day, 3-4 days per week. Plus living in the country, listening to radio, tv, etc. I think the adult will win. Even if you compare with an older kid up to maybe 13 years old, but then it's cheating because that kid has been learning the language for more than 5 years, even if you increase the "start learning definition" to age 7.