Youngster's pilot card says: Friendly TIE fighters at Range 1-3 may perform the action on your equipped (EPT) Upgrade card.

What would happen if Youngster equipped Crack Shot?

Would it be discarded the first time another TIE fighter shared it?

Or would it remain as long as Youngster himself didn't use it?

Crack Shot is not an Action, no?

Younger only shares EPTs with the Action: header (eg. Marksmanship), so Crack Shot won't work with him at all.

Youngster's pilot card says: Friendly TIE fighters at Range 1-3 may perform the action on your equipped (EPT) Upgrade card.

1. What would happen if Youngster equipped Crack Shot?

2. Would it be discarded the first time another TIE fighter shared it?

3. Or would it remain as long as Youngster himself didn't use it?

To answer your questions as written (I've added numbers):

1. Youngster would have Crack Shot equipped. His ability will never trigger because Crack Shot is not an "action header" upgrade card.

2. No other TIE Fighter may ever share Youngster's Crack Shot, see above.

3. Yes, Crack Shot will remain equipped until Youngster himself uses the upgrade card. (Unless you're playing with the old damage deck in which case you may lose the upgrade card due to the "Injured Pilot" critical hit.)