The subject is rife with many pitfalls. Survivor bias is well known in more data friendly sports, whereby only the best older players remain active later in their careers and so elevate the apparent abilities of more mature players.
An approach that attempts to account for this bias by measuring the yearly change in ability has drawbacks, most notably even the decision of which metric to chose in a fledgling data environment, such as exists for football.
Even more fundamental is how to express a side's age profile once we have settled on which parameters to measure. A sport that mixes tactical awareness, skills and physical fitness typically sees a player peaking in their late 20's.
In the case of football, different positions appear to have slightly different peaks, keepers having the most longevity and this has been covered elsewhere in this blog.
Average age, adjusted for playing time, is an obvious figure to quote to describe a team's age distribution. But this does invite misconceptions. For example, a team equally packed with youngsters and veteran players may through pure chance have a weighted average age that coincides with the accepted peak of around 28, yet without including one such player in their lineup.
A graphical representation adds more information, but the absence of numbers hinders the ability to quantify the impact of a changing age profile on a team's fortunes.
Manchester City's route to Premier League success was fuelled not only by money, but also a squad that was talented, but also played either approaching or at their peak age.
If we ignore positional variation and for simplicity assume that a Premier League player peaks at between 28 and 29, the plot above clearly shows how the age profile from City's initial Premier League winning squad moves from being predominately comprised of players approaching their peak to players at their peak when they regain the title in 2013/14 and then began to consist of players moving away from their peak during 2014/15.
As an extrapolation I've aged the squad by a year for 2015/16 and plotted how the profile would change to one consisting of a majority of players who would be likely past their prime if the share of playing time was similarly distributed in the upcoming season among the same players from the 2014/15 season.
The various graphs illustrate well the changing profile, but comparative figures would also be useful.
One simple way would be to chose a peak age and then see how far removed the weighted playing time is from this ideal peak for each age group of players. For example, an ever present 20 year old would be further away from the chosen peak age than a similarly, ever present 26 year old.
Under this scale, a side consisting of entirely peak age players would score a cumulative zero, as all minutes played would fall to peak aged players.
Manchester City's 2011/12 team score 2.5 under this method to measure the weighted proximity of their players appearances to the peak age of 28-29. Balotelli contributed 1331 minutes as a 21 year old and at the other end of the age scale, Kolo Toure played 852 minutes as a 31 year old.
As the squad converges towards their peak age by 2013/14 the arbitrary score of 2.5 has fallen to 1.1, reflecting the compression of the distribution around 28-29 years, but begins to rise to 1.7 during 2014/15, as more players move away from their peak. If 2015/16 followed the extrapolation, with no fresh blood added, City's proximity quotient (for want of a better term) will hit 3.1, compared to an average of 2.6 for the previous six Premier League champions.
Of more interest is to use this number in conjunction with a figure to describe the proportion of players approaching the peak age to those who have passed the peak age.
In 2011/12, City's weighted playing time that was outside the peak age was made up predominately of younger players. The ratio of weighted playing time for young players to those past their prime was 9.7. By 2013/14, younger players still narrowly predominated, but the ratio had fallen to 1.3, indicating near parity.
2014/15 saw City fielding a raft of 30+ year olds and relatively few minutes allocated to players aged younger than 28 and the ratio of younger non-peak players to older ones fell below one to 0.36. Older players now predominated outside of the peak age group. (Title winners average a ratio of 3.6, indicating a degree of planning from within the current playing staff).
So, as the plot for 2014/15 illustrates, City's squad has aged, moved away from the peak age and has very few replacements within the current set up.
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