The possible explanations:
1. Writernim is genius, the title "Monster" refers to reverse-aging hero who keeps getting younger and younger and so even in his mid-30s he still looks like in his 20s. Kinda like Benjamin Button.
2. All 30s something actors are currently busy, don't have time to do 50-ep drama, don't care about the writer or the script/synopsis is shit. So the producer left with no choice other than young Seo Kang Joon.

Don't get me wrong, I adore SKJ, esp now as Baek In Ho, but I'm not even sure if he'll be able to carry this drama. Esp since this seems to be male centric.

I like Lee Yeol Yum and Lee Gi Kwang. They could make cute couple. I dont dig Seo Kang Joon.

I don't think SKJ is up to be the hero. Maybe yes, he was offered to be the lead, but I think he will become the hero brother or something.
Such as reply 88 (sigh), which is the four guys cast as lead..

i highly doubt SKJ is gonna take the offer considering the length and the story of the drama (and this ridiculous 'younger cast' consisting of a guy that is actually 3 years older in real life?????). Plus criticism over his acting in Hwajung just starting to quell as he is starting to gain much better reviews over his performance as Baek In Ho. I love the guy but he just isn't leading man material yet and i doubt he'd want to take on a role that he isn't prepared for yet.

I wonder why Seo Kang Joon pushed him so much. Cos his idol group is dubbed as idol actor and there other 4 members also that can act. Especially the one that act in drama Pride and Prejudice. So is he gonna be the male Seolhyun.

If Lee Ki Kwang confirmed for this drama. Does it means that he will do drama Mrs. Cop 2?

I don't mind Hwang Jung Eum doing drama after drama. After all, that is her job as an actor. It's better that she still got offer to do drama than need to beg to acting.

Maybe they think Lee-Kikwang looks younger than SKJ?
LOL

this is kind of hilarious...anyways looking forward to the teenage years portion of the drama.

This is like one hot mess waiting to happen lol.

lol math be damned, I love SKJ but I agree with all of you, he is not ready for that role yet

For the greater good I'mma need Seo Kang Joon to decline the role and give it to a better actor!!

That said, I can't wait for my Kiki to show people he's more than just a baby face cutie!!! I'm ready for all his angst!!!

I'm not gonna surprise if they add another actor as the older main lead, it's like my love eundong which the male n female lead act by 3 generation actor n it become realisticn the story span in 20 years too, i like that idea

Seo Kang-joon is charming my socks off in Cheese and his character (along with Seol) are my favorites. The more I see him, the more I'm impressed so maybe - just maaaybe - he might do well here. I dunno.

But this weird casting is just convincing me all the more that the media probably got it all wrong. The production is most likely courting him for something else but reported incorrectly. At least that's what I hope because if it's what it seems, that names negative sense lol

already sounds like a trainwreck in the making.pass!

I feel conflicted with SKJ, maaaaybe... but I don't he's ready
I think HJE will pass on this, she's 2 drama last year plus one in between 2014-2015, beside she 'll has a husband, her priority might change and she'll get married+honeymoon,

I've read somewhere they already cast a 'hallyu star' to play the lead, but they didn't mention any name to that. So unless Seo Kang Joon is considered a hallyu star, I don't think he's cast as the main lead.

Why bother even having a separate actor for the high school days? I've seen actors in their late 30s playing themselves in high school flashbacks...

That really makes no sense. Lol.