Is this Cap fits for 21 years guy?

Yes. It's one size fits all with a pull strap in the back to adjust the fit.

Is it authentic or made in China?

Certainly not authentic!!!!Does not even look like it was made in China as a good knockoff. It looked used and faded when I recieved it. And floppy structure. Please don't purchase it. Save your money.

Is this cap stiff? I'm looking to replace a cap I lost from several years ago that was faded and soft - no structure to the crown.

These are the Unstructured Slouch Caps by 47 Brand like the faded version you had years ago. The only difference is that they make these hats in a Navy Blue Not Faded anymore.

how do you measure your head for a fitted hat

I never measured my head before so I don't know exactly how to do it. The way I did it was by going to a local hat store like a Lid's or sporting good's store and try there hats on first. Sometimes I buy it at the stores I am at depending on the price and deals the stores are having. Over time I just knew what size my head was to get the right fit. Sorry if I didn't answer your question. Try going to a store and try some fitted hat's on like what I use to do. Hope that helps.