Capello asked Rijkaard about wanting to sign a 16/17 ? year old Messi at HT after seeing Messi running circles around that Juve team (and Juve was pretty good in 05/06).
The only major concern about Messi was that he was very injury prone when young (similar to a Robben). So, in that way, it's a surprise his injuries really lessened after 2007-08.
I think there are certain players you can just tell are generational talents from the get go. Pele, Maradona, Cruyff, R9, Romario, Messi etc.
CR7 is actually the anomaly among the all time greats in that he was't a generational talent from the get go (he was obviously still mega talented), but managed to build his game all the way into his prime to reach the absolute top tier.