Since the introduction of the football aging curve, the Goalimpact is a direct function of age. Therefore, it should come as a surprise if we see a clear relationship between Goalimpact and age in a xy-plot. However, there are still some noteworthy feature to report.
|Each dot represents a non-goalkeeping football player. The x-axis is the players|
age at the 1st of July 2014 and the y-axis his Goalimpact at the same time.
Among the youth players (blue area) we see three thick vertical lines. These are caused by the Relative Age Effect (RAE). Football teams favor players born early in the year, because these are relative old compared to their team mates and opponents. This makes them have a physical advantage. As this advantage will disappear once the players are fully grown, it is a short sighted and ineffective way of player development. Rather than focusing on developing the most talented players, teams seem to focus on winning the current youth competition and play predominantly players that incidentally were born in the early months of the year. They do so, despite the fact that, according to Goalimpact, the players in the later months often are more talented. We didn't investigate it in detail, but from the chart it looks like that it were mostly the youth players that drove with the RAE ticket that didn't make it to the senior football. It makes intuitively sense as their physical play no longer works in adult football.
|The Relative Age Effect is strongly visible until the age of 19. Thereafter, however,|
it is much less pronounced. We conclude, that many players that were favored by
their relative age in the youth league, don't make it to senior football.
Most of the players in the database will never be good enough to really compete for the UEFA Champions League (UCL), A Bundesliga club aiming for UCL qualification will need an average Goalimpact of 110+ in the team. But to pass the round of last 16, this will not be good enough. The top teams will aim for players that are at least 120+. The area with those players is marked orange. Only few players are in that range and hence the clubs can expect to pay scarcity prices. This is especially true for players that are expected to stay in that range for many years, either because they are young (Thomas Müller) or because they are so extremely good that they are expected to stay above the limit for many years to come despite aging (Xabi Alonso).
Even the best players eventually decline in performance due to age. There is no fix limited when a player will stop playing. Besides the playing strength this is influenced by many other factors such as his health, his wish to go on playing, and the need of his current club. What is notable, though, is that players often drop out at a lower Goalimpact level than the marked at the green area. We assume that this is caused by different factors. Partly, this due to a bias in the aging curve. The aging curve treats all non-goalkeepers equal. In many tactical setups, however, defenders are less prone to age-related decline in performance. E.g., if a team doesn't play a high line, the reduction in speed will not influence the quality of a center back as much as striker that needs speed to counter. Hence, some defenders may actually be better than indicated by their Goalimpact. We may distinguish the aging curve for non-goalkeepers into tactical positions at a later stage, but it is not as straight forward as it seems. Some players, like Kevin Großkreutz, play many positions and it is unclear which aging curve to use for them.
Another reason why players linger around with a Goalimpact below the value that teams would accept for young players may be that older player can be still every effective but can't take 90 minutes at full strength anymore. They then might be still be very useful as substitutions or backups (e.g. Claudio Pizarro).
To summarize, a scatter plot of player age vs Goalimpact reveals that players need to have a Goalimpact above 80 to be professional football player. If they are pro already, they may stay in business a bit longer below that level. Below a level of 70, however, many drop out. Even today, youth football is driven strongly by the Relative Age Effect. The data suggests that many players that benefited from their relative old age do not manage to take the step to senior football where this advantage is gone. Football clubs could improve the efficiency of their youth programs substantially by selecting their players according to talent rather than relative age even though it may cause their youth teams to produce less victories.