A funnel, not a straight line
Academies are organised as a series of age-group squads, and at almost every transition between them a portion of players is released rather than promoted. This filtering happens for practical reasons, since clubs cannot retain every promising nine-year-old through to adulthood, but it means the path to a first team is better understood as a funnel with several narrowing points rather than a single ladder climbed steadily upward. A player can be excellent at thirteen, released at sixteen when the filter tightens around physical development, and still go on to have a significant career elsewhere in the game.
Timing of physical growth matters more than expected
One of the least appreciated factors in academy selection is the uneven timing of adolescent physical development. A player who matures early can dominate age-group football through size and strength alone, while a technically gifted but late-developing player can be judged, incorrectly, as lacking the attributes to progress. Good academies try to account for this by tracking technical indicators separately from physical ones, but the bias toward early maturers remains a well-documented feature of talent identification across the sport, and it is one reason player development at younger ages is such an inexact science.
Where academy graduates actually end up
The overwhelming majority of academy players who go on to have professional careers do so away from the club that trained them, moving to smaller clubs where first-team opportunities arrive sooner. This is not usually treated as failure by the academies themselves, since producing a player capable of a long career elsewhere still validates the coaching and infrastructure involved, even if the eventual first-team squad photo at the parent club rarely reflects the scale of the development work that happens well below it.