As stories go on, writers find a need (or opportunity) to take the viewer backwards or forwards in time from our primary protagonists. It's a difficult job for the casting director all the way through to the chosen actor themselves, because most of the people watching are watching because they like the character whose age is being played with.
And in today's world of the internet and privacy disappearing from celebrities and just plain duration of careers, it's easier and easier to hold up the actual right answer for the de-aging scenarios or the that's-not-what-the-new-actor-looked-like-at-this-age-though objections.
So—what do you do? A given face doesn't automatically age into another—sometimes it's hard to see the young kid in the face of the adult. I don't think it should be the crew's job to duplicate baby pictures, but to best find someone whose features are acceptable (Where is your mole, Colin? You could have been perfect!) and more importantly—who can pull off the same character in a different body anyway (I'd like Enver Gjokaj to play the young me, thanks). Oh, and the writers better play fair and have the dialogue do a decent part of the job.
One of my favourites (clearly, if you've gotten this far) is Colin Ford as young Sam Winchester. It's a decent physical match, and the actor is really diligent about patterning himself after what we've seen onscreen for so long—I do seem to remember him interviewing Jared to refine his role, which is endearing as well as affecting.
So—who's yours Favourite or most hated? Older too, in those scenarios where the studio forgoes dodgy makeup and fake hair...not that I'm thinking of any particular failure especially...