I used to be just carefree; I thought I was young, invincible.

我曾经无忧无虑，没有责任感，认为自己十分年轻，战无不胜。

youdao

There was an invincible army that gave young people a proper sense of discipline and patriotism.

BBC: USSR to Promised Land

Many of these groups come across as young guys that think of themselves as invincible crusaders promoting their own version of vigilante justice.

FORBES: Hacker Group Raids Fox.com, Targets FBI

Here is the specific problem: Insurance companies worry that young people, especially young men, already think they are invincible, and they are bewildered about the health-care reform in general and exchanges in particular.

WSJ: Ezekiel Emanuel: Health-Care Exchanges Will Need the Young Invincibles