Be thankful for your young genes, there really isnt much you can do

I have a beard guys. Or almost a stubble.

my question is more like how to look like my age, I am 21 but I look like 16

I'm the same. I have chubby cheeks and I'm small (I'm 17) and get mistaken for like 14

That will be an opposite of a problem after a decade. In the mean time grow a short beard.

Why would someone want to look old?
Well, grow a beard, dress formal, and start listening to classical music I guess.