翻译“尽管他很年轻，当他的行为就像一个成年人一样”

Despite his youth (adolescence), he acts (behaves) like an adult.
Despite his youth, his action (behavior) is like an adult.

Though he is very young,yet,he behaves like an adult.

He behaves like an adult despite his young age.

His behaviors as an adult despite his young age .

despite he is at a very young age , but he is behaving just like a adult.
个人感觉用现在进行时比较恰当