can a guy in his 20s pull off the cardigan?

You are drastically over thinking this. It's a sweater with buttons. 

There's a fine line between sharp GQ guy and old man territory. PM member reevolving for more info.

probably not, but as you ask certainly not. You need to have confidence to wear a number of things, if you need to ask, then you do not have it.

Why wouldn't he be able to? I don't get this. Of course you can wear a cardigan. The rest of your outfit and the fit will determine whether it looks out of place or not. If you have to convince yourself, then it's probably not going to work. You'll feel and look like you're wearing a costume.

I'm early 20's and I wear a nice, thick navy cardigan all the time. I always get comments that are either explicitly positive or kind of giving me a ribbing (eh?) for it in a joking way.

I find when I try to "pull off" a sweater, the shoulders get a bit stretched out, and so I aim for more of a "careful removal."

 

Seriously: I don't own one yet, because I haven't found one that fits properly -- the slimmer-fitting of them don't work with a shirt under them, and the baggier have low armholes and do look a touch old-mannish. That's my problem, not yours, and I bet you could make the right one for you work just fine.

i see several teenagers wearing a cardigan. I don't see anything wrong about it.

Yes, younger men can wear a cardigan sweater without raising eyebrows. In fact, today I just canceled my subscription to Esquire in light of their almost fetishistic devotion to the sweater in general and *OMG, Daniel Craig in a cardigan!!!* in particular.

If you fear looking old-mannish, buy from a fashion forward brand (Uniqlo on the cheap or S Schneider to break the bank).

And +1 on the overthinking.