I have read that a person is not judged by the Heavenly Court until he is 20 yo. So any sin he has done does not count against him in Heaven. I have also read that all of a person’s sins are forgiven when he gets married, is this true, and what is the source of this?

It is written in Kabbalistic sources that the Heavenly Court does not punish for sins done before one is 20 yo, unless one continues to sin, in which case one is punished for both. One fasts on the day of the wedding. This and the wedding itself forgives the sins. Rashi on the Pasuk Machalat (Bereshit 28,9) brings this. Rabbi Yitzchak Grinblat