"青年冒失莽撞" can be translated into English as "young people being reckless and impulsive" or "the recklessness and impulsiveness of youth".
Here are some more context-specific variations:
1、 "The reckless and impulsive behavior of young people" (突出青年冒失莽撞的行为表现)
2、 "Youthful recklessness and impulsiveness" (以名词形式简洁表达)
3、 "Young individuals' tendency to act rashly and thoughtlessly" (强调倾向性)
选择哪个表达取决于具体语境和想要强调的重点。