Real Madrid's squad isn't exactly young, and this becomes clear when you look at the most expensive eleven made of players under the age of 23. Here is what it would look like, as per Transfermarkt.
Goalkeeper: Andriy Lunin (€2m);
In total, this team is worth €337.5m. As you can see, there aren't enough first-team players to make a full line-up, so we had to 'borrow' five players from the reserves.
Also, for the record, Luka Jovic turned out to be 13 days too old to make this line-up. The Serb is valued at €20m.