When it comes to the national team of any country, age counts for nothing as only competence and ability matter. This explains why the likes of Edson Arantes do Nascimento, known as Pelé and Kylian Mbappé became World Cup winners as teenagers for Brazil and France respectively.
Ronaldo de Lima of Brazil and Argentina’s football wizard Lionel Messi both went to their first World Cup as teenagers even if they had limited roles to play.

In the 1998 FIFA World Cup, Micheal Owen justified his place in the English squad coached by Glenn Hoddle as a then 18-year-old with his iconic displays especially against Argentina in the second round before England lost on penalties. Recently, Nigeria seems to have been caught in the web of deciding whether Samuel Chukwueze should play for the Flying Eagles in Poland at the Under-20 World Cup or for the Super Eagles at the African Cup of Nations holding in Egypt.

https://qwenu.com/2019/05/11/is-samuel-chukwueze-too-young-to-play-for-the-super-eagles-of-nigeria/