Of the many factors that might explain a successful NBA defender, his age is one worthy of consideration. To measure it I collected data from basketball-reference.com on full season stats for every NBA player from the 1990-91 season until the 2011-12 season. Because qualifying a player as a 'successful' defender demands subjectivity, I also categorized players based on if they were voted 1st or 2nd All-Defensive Team in a given season or if they were not voted to an All-Defensive team.
A few things to note about the All Defensive Team. First, while this seems like a reasonable measure of defensive aptitude, it's worth noting that head coaches vote for the team. As a result, there could be bias in the voting process. For example, if a coach harbors ill will toward an opposing player for personal or competitive circumstances he may consciously or subconsciously disregard that player when casting his vote. There's also the issue that popular players may possess greater relevance in a voter's memory. As a result, coaches might continually vote for Kobe Bryant not because he was a top ten defender in a given year, but they recall his name quicker than less popular and less offensively capable players or because Kobe's defensive achievements from prior seasons affect perceptions on his defense for the current voting season. In other words, this isn't a perfect measure but it is reasonable. Also, most seasons ten players--five for the first team and five for the second team--are selected to the First and Second All-Defensive Teams. In other seasons, like 1998-99, 2004-05 or 2005-06, there is a tie in voting so 11 players are selected.
Before introducing some basic analysis it's important to identify why age would affect defensive ability. On the one hand, successful man-to-man defense (which is the dominant scheme of the NBA) demands exceptional athletic ability because the defender must react to the cuts, bursts and moves of another world class athlete. As a result, we might expect younger players to have an advantage as defenders because youth better serves athleticism than the time, wear, and injuries experienced by a rugged veteran. On the other hand, defense also demands intelligence and guile to read an opponent's tendencies, to process and predict offensive schemes, and to understand how to best neutralize an opponent's offensive strengths. These are qualities one might expect a veteran to possess from his year's of competition and, thus, would suggest that older players serve as better defenders.
Below are two charts. The first shows the mean and the second shows the median (or middle age of the group) of all All-Defensive First and Second team players and all other players (i.e., those that weren't voted to an All-Defense team)*. The top chart on average age shows that for every season since 1990-91 except for the 1999-00 season the average age of the All-Defense team was greater than the average age of the rest of the NBA.
A few things to note about the All Defensive Team. First, while this seems like a reasonable measure of defensive aptitude, it's worth noting that head coaches vote for the team. As a result, there could be bias in the voting process. For example, if a coach harbors ill will toward an opposing player for personal or competitive circumstances he may consciously or subconsciously disregard that player when casting his vote. There's also the issue that popular players may possess greater relevance in a voter's memory. As a result, coaches might continually vote for Kobe Bryant not because he was a top ten defender in a given year, but they recall his name quicker than less popular and less offensively capable players or because Kobe's defensive achievements from prior seasons affect perceptions on his defense for the current voting season. In other words, this isn't a perfect measure but it is reasonable. Also, most seasons ten players--five for the first team and five for the second team--are selected to the First and Second All-Defensive Teams. In other seasons, like 1998-99, 2004-05 or 2005-06, there is a tie in voting so 11 players are selected.
Before introducing some basic analysis it's important to identify why age would affect defensive ability. On the one hand, successful man-to-man defense (which is the dominant scheme of the NBA) demands exceptional athletic ability because the defender must react to the cuts, bursts and moves of another world class athlete. As a result, we might expect younger players to have an advantage as defenders because youth better serves athleticism than the time, wear, and injuries experienced by a rugged veteran. On the other hand, defense also demands intelligence and guile to read an opponent's tendencies, to process and predict offensive schemes, and to understand how to best neutralize an opponent's offensive strengths. These are qualities one might expect a veteran to possess from his year's of competition and, thus, would suggest that older players serve as better defenders.
Below are two charts. The first shows the mean and the second shows the median (or middle age of the group) of all All-Defensive First and Second team players and all other players (i.e., those that weren't voted to an All-Defense team)*. The top chart on average age shows that for every season since 1990-91 except for the 1999-00 season the average age of the All-Defense team was greater than the average age of the rest of the NBA.
The second chart shows that the median age of All Defense players is greater than or equal to all other NBA players in every season except 2009-10 when the two groups had the same median age. Therefore, looking at data from the last 22 NBA seasons it appears that defensive aptitude, at least in terms of being measured by whether a player was voted first or second team All-Defense, increases as players age and gain experience. (Of course, only up to a point as All-Defensive Team players' age seems to hover in the late 20s/ early 30s. There likely comes a point in the mid-30s where despite the knowledge and guile of these defenders, age compromises athletic ability to too great a degree.)
*The reason for looking at the mean and the median is because very large or very small values can skew the mean but won't skew the median. In other words, if you took a sample of five players and their ages were 24, 25, 25, 26 and 41 then looking just at average would make us think that the average age of the group was greater than the rest of the population of players because of the anomalous 41 but the median, 25 in this example, gives another perspective on the 'central' age. Because NBA player age is usually at most a total variance of 20 years between the youngest and oldest player, anomalous ages would only affect small groups, like that of ten or eleven players voted into All-Defensive Teams.
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