Pep Guardiola may believe Manchester City remain teenagers compared to the titans of European football, but a 7-0 thrashing of a hapless Schalke to storm into the Champions League quarter-finals was another sign the English champions are growing up fast. Already leading 3-2 from the first leg in Germany three weeks ago, a quickfire double from the evergreen Sergio Aguero ensured City would not suffer the sort of collapse that has cleared Real Madrid, Paris Saint-Germain and Atletico Madrid from their path towards a first ever European Cup. Thereafter, it was a procession with even a real teenager on the scoresheet as 18-year-old Phil Foden came off the bench to get his first Champions League goal. The other goalscorers are hardly veterans. Leroy Sane, 23, Raheem Sterling, 24, Bernardo Silva, 24, and Gabriel Jesus, 21, showed that City have assembled a squad built to make Champions League quarter-finals routine over the next decade. Yet, there may be no better time than the present to finally conquer Europe with the threat of a Champions League ban looming in a UEFA investigation over alleged transgressions of financial fair play rules. “When yesterday (Monday) I said we are a teenagers team, I didn’t…