I think that the peak is between ages 24 and 25.
Those players are still somewhat trainable, so you can fix smaller flaws, they will stay on top of their game for years and can be sold again when they become older.
Größter Knecht aller Zeiten aka His Excellency aka President for Life aka Field Marshal Al Hadji aka Lord of All the Beasts of the Earth and Fishes of the Seas aka aka Conqueror of the Buzzerbeater Empire in Europe in General and Austria in Particular