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

Young people should never fail an invincible courage.

年轻人决不应该缺少大无畏的精神。

youdao

When you are that young you feel invincible.

是我们所向无敌的时候了。

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