Flashbacks are great for storytelling purposes - examining the early days of a character to see how far they've come OR how little they've changed. The problem is that (in TV and film), trying to get an actor to believably portray a much younger version of themselves can be tricky, since they have to appear younger for the audience to be able to continue to suspend their disbelief and stay invested in what they're watching. And that's why Hollywood has used these techniques over the years to magically make actors appear more youthful than ever!
You know what young people have? Long, floppy hair! They keep it long to hide their faces, which are mysteriously as old and withered as ever.
Dexter's adult Justin Bieber haircut - never forget.
Nothing says "I'm a believable young person" than having Barbie-esque hair, which is a thing that lots of people definitely do (when being a young person IRL).
Remember when a character was younger and thought a clearly glued-on mustache or beard was a good look? Bad mustaches - the mark of youth, and definitely not 42 year old stepdads.
You know how you can tell a young person? Disturbingly smooth skin, like they're being airbrushed in real time.
You know how we exclusively cast attractive thin people on this show and 10 years ago they looked virtually identical because they have to maintain this weight and appearance for eternity if they want to have a career? That means their younger self looks pretty much the same as their current self, which isn't too interesting. But what if they were, like, FAT or something. Can you imagine? A thin attractive person being fat? It'll both establish that they're younger by looking different, and reinforce that fat people are hilarious to look at.
Ya know, it's just not worth the time or effort to try to make this obviously older actor appear to be several decades younger. It's nice to have that continuity of character, but we can't realistically pull it off with any believability. So let's just cast a terrible who looks and behaves nothing like them - but who is younger. Cool? Cool.
Just, like, say they're younger or whatever. It's a goddamn flashback and we'll add context clues in the dialogue so the audience understands that - they're not THAT stupid. They'll understand what's going on without ridiculous wigs or shitty CGI...right?