"青年冒失莽撞" can be translated into English as "Young people are often reckless and impetuous" or more naturally, "Young people tend to be impulsive and rash."
Here, "reckless" and "impetuous" (or "impulsive" and "rash" in the second version) convey the meaning of acting without thinking or with excessive boldness, which aligns with the Chinese phrase "冒失莽撞" (mào shī mǎng zhuàng), meaning "rash and reckless" or "impulsive and hasty."