Mortimer would just be incredibly saddened, really. He knows no amount of speeches would stop that 18-year old about to do something really stupid, but he'd love to prevent all that suffering from happening if he could. Said teen self would look at his well-kept wizard robes and think he's made it as a court mage, though, which would be very far off from his current state.
My scruffy courier would probably joke about and banter, and play some piano accordion with him. Before he left, he'd say something along the lines of "You're goin' tae lead a very strange life, lad... But it'll work out. Even when it feels like it won't. Mind your Professor Basil now. He's full of good wisdom."
Considering I have a group I think it should be fair to discuss all three even if Vernon is the MC.
Vernon has a much more easy going life up until a certain point... His older version would probably hug his younger self and cry while talking about all the things he is sorry he did, while his younger self is sitting there consoling him not knowing what the heck is going on.
Grey would probably lecture his younger self... He was quite the edgy teen (even if he had reasons to be) so he would probably spend a good while telling his younger self some of the lessons he learned over the years. And his younger self would probably not listen and tell his old white ass to piss off.
Azreth ... Well... Azreth's only growth ever since he was a child was that he is a slightly less naive and a lot bigger (like physically) than his younger self. Azreth grew very little mentally... He is hornball and his younger self was even hornier because of you know 'teen spirit' and because dragons in my story undergo a much faster development... So yeah, you can probably guess what happens when he meets himself.
This might be cliche, but Aeri would totally give her younger self a hug and tell her things would get better for her. Something like "Hey, you're not always gonna be stuck here, and you're gonna make friends soon. You have something to look forward to, so just keep your chin up and keep on waiting for your moment."
A bit of off-topic though: Most of my characters may not even recognize their own looks, not to mention their younger selves-- unless they really take time to glance themselves onto large still water reflection. (Maor's papa might had recognized that.) There's no invention of mercury mirrors either, even if there's copper/silver mirrors they're a thing of luxury, not common in my fantasy villages.
Most definitively, they will see their younger selves as some other persons.
Since my MCs are 3
sorry I was tempted to post their babyselves cause omg this thread made me emotional about them
Ira is not a thinker, so he probs will just jump to try to help his younger self with the same thing in an attempt it works this time
Liar would try to trick his younger self to move out the country with the whole family lol
Caelis will just give an advice to himself in hopes to change his mind
I have an ensemble cast, so heres a little for each one: (SWEAR WARNING)
Agony: grabs younger self by shoulders "you stupid little shit! Do something with your goddamn lie before the fuck'n world ends you idiot!
Thesis: "It doesn't get better. Only weirder."
Duenna: "Oh... so young... so oblivious."
Motif: "Hey! Your about to do a lot of things, a lot of big stuff. DON'T! Don't do any of it, just don't! Whenever your about to do something, just think about it. And then probably don't do it. DON'T DO IT!!!
Sequel: Starts crying
I think Joey would melancholically watch from a distance, because he knows he can't really stop the death of his best friend, merely delay it and he wants his younger self to experience life to the fullest up to that point. He would leave a little note though with: "give Simon another chance."
Simon would probably run up to his younger self en tell him that something bad will happen soon and that no-matter what, he should be there for Joey and he should not close himself off. Younger Simon would probably just look at him really weirdly and think how much of a loser he is in the future haha.
My mc Saul Torres would probably try to play with his younger self and then try to keep that he's his future self a secret in case it creates a change in his history for the worst. Dx he's already dealing with enough problems as there is.
Funny, since I did plan some kind of paradox happening at some point, but if we put my MC Josi from the beginning of her saga with a version from all the way at the end of the fourth side by side and let them meet, she'd probably also give her some really encouraging words, since there'll be a lot of things that'll try to push her down over the course of the story.
And something along the lines of "Mew is your snack-thief. It always is, never trust its excuses."
My MC, Kazimir, would probably try to prevent the trauma from happening to him as a kid. I can see him coming up with all kinda of crazy ideas to keep his younger self safe. (Being kinda vague cuz of spoilers) I'm sure Kazimir's younger self would think of him as an adult as a weirdo.
Honestly, I think dan would cry or be depressed if he saw his past self. Dan's past life was pretty traumatic for him.
My MC is dead and can't remember the time when he was alive, so he would probably just pass by without noticing XD
My MC is only fifteen and usually puts other people first (even if that’s his younger self in this case). 
He would probably step in the room to warn himself about the future, but end up playing games or telling jokes instead, because all little Gen needed was a friend. 
Desquicio would pat young Desquicio in the back and tell him something in the lines of "You're doing great, buddy. Keep going and don't hold back"
Claudia would look at her younger self from a distance and cry inside.
The madman would look at his younger self and have no idea who this kid is. His younger self, however, would understand who this older person is and feel very very worried about the future.
Luck would just endlessly apologize and cry to himself.
Saza would probably just give his younger self a lot of love, validation, and "do not listen to them"
Well... he grew up during a war so there's a limit to how innocent we can get here. If we're talking before the whole messed-him-up-for-life event then he'd be around ten or younger. I think Kovreil would just want to play around with his younger self and make sure he enjoyed their time together. He'd probably be uncomfortable since thinking of his childhood always reminds him of things he lost, but if he could give his kid self a fun time with some smiles and laughter it would mean a lot to him (and he'd probably cry afterwards to be honest).
