Sforno on Numbers 14:23/ספורנו על במדבר י״ד:כ״ג

1 וכל מנאצי לא יראוהו, also their sons who at this time had not reached the age of 20 but would be among those who disdain Me in the future, will also not be granted the privilege of seeing the Holy Land. Examples of this decree were the men who died during the uprising of Korach and had been under the age of 20 (at the time of the Exodus), as well as the people who died at Shittim 38 years later because of their involvement with the Moabite women, as well as the people who died before Moses could erect the copper snake. The people concerned had not been involved in rejecting the land of Israel at the time when the spies made their report as they were under age. (compare Numbers 21,6 and 25,9)