I'm 20yrs old and my penis is 10cm long when erect. Is it good.

Penis anatomy. This is shorter than average for a man. 10 centimeters equals 3.9 inches. The average erect penis is 4.5 inches. So, that means that 50% of men are shorter than 4.5 inches and 50% of men are longer than 4.5 inches.
I don't know. If it's good for you, then it's good for me. Besides, it's the only penis you'll ever have, so it will have to do no matter how long it is. Size doesn't matter, ability does.