Many will intuitive believe that the peak of a professional sportsman's career arrives around his mid twenties especially in a physically demanding sport such as football. However, providing evidence for this claim is often more difficult. Managerial selection that favours mid twenties players at the expense of either extreme provides some confirmation, but attempts to quantify this is fraught with problems.
Counting or averaging headline performance indicators, such as goals scored by strikers often produces a pleasing curve that obligingly peaks around the expected age groupings. But selection bias is an ever present danger, as youngsters deemed not yet good enough to start and veterans, no longer good enough to merit selection are inevitably absent. Exceptional youthful and aged talent such as Messi and van Persie respectively can easily dominate the apparent contribution of usually under represented age groups and, depending on how the data is presented, created false peaks where none generally exist.
Also because, almost uniquely, football has a global reach, some exceptional players can leave the EPL, not because they are declining, but because they are exceptional. Ronaldo's goalscoring exploits as a 20 something at United would hang heavy on that particular age group, but wouldn't be represented in a cumulative approach in the later age grouped performances, because his is now in Spain.
One way to try to eliminate the influence of statistical freaks is to plot the change in key indicators. Messi's scoring achievements may be big enough to skew an age related plot based on cumulative totals, but for him to continue on an upward curve, he, as well as every other player in a sample need to try to better their previous year's performances.
Whilst a player can generally continue to improve a rate statistic, such as goals per minute played, he can be considered to be still climbing towards his statistical, age based peak. Once that indicator begins to consistently fall compared to previous achievements, then he can be considered to be in decline. Although this shouldn't make his departure from his league of choice imminent. A declining Messi may well be superior to most other players in La Liga. Inevitable decline should be balanced against how high the player has set the bar by his earlier performances.
However, this approach also flounders because fluctuating player statistics often have various likely causes. A player may see his goal per minute rate fluctuate for reasons other than just maturing or ageing. Individuals regularly move between clubs and different sides may employ different attacking formations, new, improved team mates may compete for goals.
High profile loanees, such Sturridge and Lukaku at respectively lower scoring Bolton and WBA scored a higher percentage of team goals than they can expect to score at their parent club. A higher percentage of a lesser total goals may not equate to a lower percentage of a higher scoring team total once they return to their parent club or in the case of Sturridge move on to a higher rated side. So the goal environment in which they play and the relative attacking abilities they are surrounded by may be expected to change individual scoring rates in addition to any expected ageing effect.
We can try to eliminate possible fluctuations to a player's goal scoring rates caused by a change of scenery by sticking with players players who remained at the same club for a period of five or more seasons. Dennis Bergkamp, who spent his mid twenties to his mid thirties at Arsenal and less prominent strikers, such as Deon Burton's five seasons at Derby.
Improvement appears to stop, and decline in scoring rate for individual strikers begins to set in just before a player hits 25. So, on average for this group, (who distinguish themselves from the rest of the EPL's strike force, randomly through their reasonable loyalty to one club), goal rates improve until 23, plateau for a year before beginning to tail off as the player enters his mid to late twenties.
Similarly, from the line of best fit, the average decrease in scoring rate for a player going from 34 years old to 35 is around ten percent of his figure recorded in the season when he was 34. If he is/was an exceptional talent, his higher base level may keep him in the league. But often by now, only the very best can fight off younger challengers or imports from abroad, who may also be in decline, but from a higher perceived level.
By comparing pairs of performance years, controlled for employer, the effect of a player carrying raw, extreme talent (even if it is declining) into less populated age brackets is avoided.
The trends though are general. Some players may prolong their scoring streak, but the league trend is often a powerful influence, eventually.
Ageing effects undoubtedly exist, but we may require the less turbulent setting of a one team clubman to be able to possibly identify evidence within the numbers. The ability to better project the likely scoring feats of players who are transferred into better or weaker teams, sometimes abroad, resulting in them playing contrasting tactical game plans is the logical first step before trying to seek evidence in the wider playing population.