Mention of Aguero is an interesting one. Here's a guy who played three seasons in the Argentinian league, 54 games 23 goals in total, 36 games 18 goals in his final season and played just two games in the Libertadores. He Moved to Atletico Madrid when he was just 18, took one season to adapt to the Spanish league, then scored 19 league goals in his second season. He also didn't make his international debut until after he moved to Atletico.
Now to compare Aguero with Neymar. Neymar has 102 games in the Brazilian league with 54 goals (so he's played almost twice as many games and scored almost twice as many goals, in a much stronger league). Last season Neymar scored 14 goals in 17 league games (a much better strike rate than Aguero in Argentina). Neymar has played 29 games in the Libertadores and scored 15 goals (much more experienced than Aguero at a continental level and a very good strike rate too). He's 21 years old now (so three years older and much more experienced than Aguero when he moved to Europe). Neymar already has 32 caps for Brazil and 20 goals (again much better than Aguero before he came to Europe, who had no international experience).
Of course Barca are a much bigger club than Atletico but if Aguero can move to Europe aged 18 with just over 50 league matches played, almost no continental experience and no international experience and become a big success then I see no reason why an older and much more experienced Neymar can't be a huge success.