This will be short and very understandable. Why we receive youngsters from our school with 18 years when in the real football they can play from the age of 16. It will be good if that is changed in this game.

Here's some Examples!!!

Messi first goal in Barcelona - YouTube
Messi was only 17 years, 10 months and 7 days old, making him the youngest player ever to score a league goal for FC Barcelona. This was just info for the goal but he signed his first professional (senior) contract when he was only 16.
Neymar made his professional debut on 7 March 2009, despite being only 17 years old.
The same with Cristiano Ronaldo and many more soccer players.
The Club can sign the player but they will be to inexperienced by scoring less goal and make more mistakes. But that will be the Clubs decision if they want to put them in the game or not.