Getting game time might be an interesting process and could be dependent on multiple factors. While every footballer would want play for the likes of Barca, PSG etc, other countries and clubs might (surely) display greater opportunities.
Below are some numbers indicating the average age of line-ups fielded, focusing on the Big 5 for Domestic league matches played between 01/01/2021 and 21/03/2022.
The youngest squads were recorded in the Croatian top division (25.3 years of age), the Mexican Liga Expansión and the Danish Superliga. At the opposite end of the spectrum (oldest squad) are: the Paraguayan top division (28.9 years of age), the Thai League 1 and the Primera División in Bolivia (not displayed).
Getting more precise about young players, the following chart displays the percentage of minutes played by footballers that have not celebrated their 21st birthday yet.
Venezuela sits at the top followed by the Danish top division declining rapidly (more than half) until Ligue 1 as the first Big Five league.
Considering 11 players in a team (and pretending there wont be a substitution in a game), there were 1.76 (U21) players in the starting line-up playing 90 minutes.
The Premier League, the Primera Division and the Serie A displaying only a quarter of that value.