Well, He is only in his twenties, anon.
The biggest reason I imagine someone with a babyface playing him though is because he was attractive at 14. If the attraction about him was based off of more masculine attributes, 14 would have been an awkward stage for him. Usually it takes awhile for them to grow into that kind of attractiveness.
However, if he were “pretty”, then it would have showed even then. Therefor, I imagine him to be exactly that. “Pretty”, but with accents of sex appeal that came with age.
I also can’t see a really masculine person having the type of breakdown he did. I know that it can happen; I’m not sexist or anything —I just simply can’t picture it. So the babyface helps with that, as well.
Also, I realize that he is very strong, but he also has to be lean as he’s a swimmer. The biggest mistake I think they could make is to get some bulky, manly person —because personally, I just don’t see it. Especially with his voice. Katniss described it as more of a purr, and I unfortunately feel like they’d go with someone husky.
That’s why I picture Frank so much. He’s small, but not too small. He has this aura of arrogance about him, but he also looks like he could play the “in love” and “mental” Finnick just as well. I won’t even start on his voice.
To each his own; This is just how I imagined him.