Most of the growth in the penis size happens through puberty, and it is safe to assume that by age 19 or 20, it is fully grown.
There are no clinically proven, non-surgical methods to increase penis size, and any claims being made to increase it should not be trusted.
To index your penis size you can take this very quick assessment that will give you your length percentile on the basis of data published in the British Journal of Urology International in 2014.
Find out your penis length percentile