In the sample sentences that you provided, B would have a stronger feeling to it. "Pretty much" is typically used to mean relatively or somewhat. So the first sentence may lead the listener to believe that you are saying that you look younger, but not by a large amount. In the second sentence, the listener is lead to believe that you look many years younger than you actually are. I hope that this helps. :)

Pretty much is very different than very :) I can say "the car goes very fast" or I can say "that car is much faster than the other one" or I can say "that is pretty much the fastest car around" I can also say "I am pretty much done with school" That implies that I am done with school, but maybe I have one or two things left to do to make it official.

"Pretty much" has the meaning of "very nearly" or "almost". So sentence B would have a stronger sense than A

Hi Yoji, saying I pretty much look younger than I am is a bit confusing. Your second statement of I look younger than I am is clearer in English it makes more sense and is more likely to be said in conversation rather than your first statement.

Thank you teacher Lize L! So, what you are saying is that sentence A is gramatically incorrect?