For other tie fighters for them to use youngster upgrade does youngster have to use his upgrade car as action?

Youngster doesn't have to use it himself, no. Each friendly TIE Fighter can use his Action: header EPT as though they were equipped with it, but Youngster himself can do whatever he likes.

But if you equip Youngster with an elite upgrade that doesn't have the " Action: " header, no one else can utilize it, and his ability is wasted.

Thanks. Makes sense.

And remember you can use it on TIE/FOs and TIE/SFs too~

One last question that just came up - if youngster is destroyed before the other T-Fighters attack in the same round, do they lose the extra attack dice since Youngster had the upgrade and Youngster was destroyed? Or, since they declared their action during the activation phase, do they still have the ability even if Youngster was destroyed in the combat phase before their attacks?

I'm assuming you're suggesting Youngster with Expose, in which case any TIE Fighters within Range 1-3 would have declared that as their action, and that ability remains active for them. Youngster's destruction won't change their actions in the same round, but it will mean they cannot use Youngster's Expose in following rounds.

Got it thanks