Original actors, within reason. Victor Garber managed a younger version of himself in his 30s in "Alias" flashbacks; however, if you were going to show him being recruited to the CIA while he was 18, you'd want to get someone else. Same would go for Lena or Ron -- with hair, makeup and lighting they can play a couple decades younger without it being incredible. But if they were going to be very young, college-aged, you would want younger performers.

And if somebody were making an Alias vid with flashbacks -- just if anybody had that incredibly delightful idea! -- there would be tons of old footage of Victor Garber and Lena Olin (and to a lesser extent Ron Rifkin) available to cull from.