America's fighter jets are not outdated. (And i have to say specifically their fighter jets, because other parts of their air force are very outdated yet unable to be replaced because they are no longer even produced.)

The sentence

Iran literally bombed a US base with a six decade old fighter jet

refers to the age of the Iranian jet that bombed the base, not a US jet being bombed on the base. More specifically it was an old F-5 sold to Iran by the US before the revolution.

This admission shows: a) that the claims that the US had neutralized Iran's entire air force were at best exaggerated, and b) that the air defenses around their bases are very poor.

There is also speculation that the story we were fed at the beginning of the war about US jets being accidentally shot down by Kuwaiti jets (which are also older models that look very similar to the ones Iran is using), may have been a cover to hide the fact that the most modern US jets are being downed by ancient technology.