Pelé became a full-time starter when he was 17, in 1958.
He was Santos´ best player in the 1958 Rio-São Paulo tournament, which - as the name suggests - contained clubs from the states of Rio de Janeiro and São Paulo. It was the closest Brazil had to a national tournament at the time, considering it included 8 of the 12 big brazilian clubs.
Like you´ve mentioned, he also played the state championship of São Paulo in 1958, which Santos won.
I consider Lamine Yamal the best U-20 footballer in the world. He definitely has the potential to become the best player of his era and I consider him better than the likes of Endryck, Kendry Paez, Arda Guler and Estevão.
But I think it´s too premature to put him in the same league of football legends like Messi, Ronaldo, Pelé and Maradona. Ansu Fati looked really good at age 16 and look where he is now.
Maybe I´m biased but the best teenage footballer I´ve ever seen is the original Ronaldo. He was the complete package both from a technical and physical point of view. His age 16 and 17 stats:
58 Games
56 Goals
21 Assists.
He then went to PSV and kept a 1 goal per game average.
His peak was really short though. Despite being great in the 2002 World Cup, he was never the same player after his injury against Lazio in 2000.