Yeah but they are young because all their players are around the same age, other teams often have at least one player who is a few years older. flamie might seem old but he's actually only 23 years old.
NaVi have an average age of 22 years, and m0nesy is only 15 years old. That's a difference of 7 years. And sure, in some teams like Fnatic, that doesn't matter, where Brollan is almost 10 years younger than the rest of the team. But a big difference there is that Brollan has a very mature work ethic, unlike your typical 18 year old. I highly doubt that a 15 year old could fit into NaVi's dynamic.