All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have not come across any evidence that 'Umar shaved the beard of a young man, but it is reported that he shaved [or trimmed] his hair because he heard that some women were saying some love poetry about him. Many scholars stated this incident like Ibn Taymiyyah, Ibn Al-Qayyim and Al-Aloosi. The whole story of this incident is mentioned in the book "The History of Damascus."
As regards the ruling of shaving the beard, you may refer to Fatwa 90928.
Allaah Knows best.