I thought UEFA uses a player's age at the beginning of the season?
A "club-trained player" is a player who, between the age of 15 (or the start of the season during which he turns 15) and 21 (or the end of the season during which he turns 21), and irrespective of his nationality and age, has been registered with his current club for a period, continuous or not, of three entire seasons (i.e. a period starting with the first official match of the relevant national championship and ending with the last official match of that relevant national championship) or of 36 months. In the context of this paragraph, the season immediately preceding a player’s 15th birthday may be counted if his birthday is after the last match of the relevant national championship but before or on 30 June (winter championships) or 31 December (summer championships), and the season immediately following his 21st birthday may be counted if his birthday is on or after 1 July (winter championships) or 1 January (summer championships) but before the first match of the relevant national championship.
UEFA 2023-24 Season:
45.11 A player may be registered on List B if he is born on or after 1 January 2002 and since his 15th birthday has been eligible to play for the club concerned for any uninterrupted period of two years, or a total of three consecutive years with a maximum of one loan period to a club from the same association for a period not longer than one year. Players aged 16 may be registered on List B if they have been registered with the participating club for the previous two years without interruption.