The age refers to the draft relevant age. For the NHL draft, a player must be 18 by Sept 15 of the draft year. Rather than listing raw age, which makes it difficult to determine who is draft eligible, we list the age on Sept 15 of the draft that follows the season in question. So, if you want to only see the stats of players eligible for the 2021 draft, you would look at players who are listed as 18 in the 2020-2021 season.
The formula is
convert_to_years(date(<season_end_year>-09-15) - player_birthdate)