Mary Young Lamont's four sons changed their names. The youngest son, Andrew, took his mother's maiden name of Young for his new surname. It is assumed that all four boys were young children because if they had been teens or older, they would have been fighting with their father and not left behind with their mother.
This is “ Group 08” at the Lamont Y DNA project:
https://www.familytreedna.com/public/LamontDNA?iframe=yresults
Actually, it seems like the boys all took different surnames.
Described a bit more here:
https://www.genealogy.com/forum/surnames/topics/young/13581/
To answer your question about why the Young name. John Lamont and about 250 other Lamonts were killed by the Campbell's in Scotland about 1640. Mary Young Lamont fled to Antrim, Ireland with 4 young children, they all chose different names fearing reprisals. The four names were Young (after mother's madien name), another was Sitlington after a creek nearby, and White and Green being the other names. I believe they were Septs of the Young name.
——
But we should make sure we have correct John Lamont linked.
It has been claimed that Andrew Lamont Young is s the son of Sir John Lamont and a wife name Mary, but I cannot find any mention of this marriage in Hector McKechnie's history of the Lamont Clan, or in the Inventory of Lamont Papers edited by Sir Norman Lamont of Knockdow.
https://archive.org/details/lamontclan12351900mcke/page/n8/mode/1up
https://archive.org/details/scottishrecordso54scotuoft/page/n8/mode...
I have supposed that the father claimed for Andrew Lamont Young may be the man named John Lamont whose name is included in the pedigree printed on page 421 of the Lamont history. However, it would appear that this man, who is said to have been killed in 1646 (as was his brother Duncan Lamont of Knockdow), had no children. Certainly, none are mentioned in McKechnie's history.
https://archive.org/details/lamontclan12351900mcke/page/421/mode/1up