Just to summarize what Stever and I discussed above, here is an updated chart
Code:
Season age career 30 33 35
1947 26.2 1.000 11
1948 26.4 1.548 10
1949 26.4 1.653 17
1950 26.7 1.657
1951 26.3 2.558
1952 26.2 2.412
1953 26.8 2.580
1954 26.4 2.553
1955 26.5 2.905
1956 26.4 3.706
1957 26.4 3.375
1958 26.3 3.917
1959 26.4 3.786
1960 26.4 5.136
1961 26.6 4.167
1962 26.6 3.324
1963 26.8 2.724
1964 26.9 5.227
1965 26.8 3.682
1966 26.6 5.130
1967 26.6 3.462
1968 26.7 2.261
1969 26.9 3.833
1970 26.7 4.864
1971 26.5 3.511
1972 26.1 3.457
1973 26.2 5.029
1974 26.3 4.553
Season age career 30 33 35
1975 26.4 4.913 52 14 5
1976 26.2 5.732 50 13 3
1977 26.2 4.419 45 12 2
1978 26.1 4.055 37 10 1
1979 26.4 4.553 45 10 2
1980 26.4 4.323 53 11 3
1981 26.8 3.855 48 9 1
1982 26.6 4.525 53 19 1
1983 26.9 5.000 51 20 3
1984 26.9 5.041 48 17 8
1985 27.1 4.978 49 15 6
1986 27.2 5.203 60 14 5
1987 27.2 4.691 62 15 4
1988 27.1 4.679 67 15 5
1989 27.2 4.315 73 21 5
1990 27.2 4.783 87 32 9
1991 27.5 6.241 90 36 13
1992 27.7 5.636 88 27 13
1993 27.7 5.328 99 37 13
1994 27.9 4.852 102 36 13
1995 27.8 6.051 102 36 13
1996 27.7 4.649 112 39 18
1997 27.4 5.658 132 67 28
1998 27.2 5.790 140 64 28
1999 27 5.887 146 69 39
2000 26.9 6.696 150 71 37
2001 26.5 7.750 155 74 39
2002 26.6 7.900 136 71 35
2003 26.8 6.403 119 59 29
2004 26.6 7.474 121 61 30
2005 26.6 6.629 117 56 26
2006 26.6 5.500 112 43 20
2007 26.6 5.197 118 40 20
2008 26.7 6.985 121 43 19
2009 26.5 6.057 113 47 14
2010 26.8 6.070 109 48 18
2011 26.6 ------ 122 46 22
2012 26.6 ------ 123 51 29
2013 26.7 ------ 121 45 23
2014 26.5 ------ 106 54 26
2015 26.8 ------ 110 53 20
As Stever mentioned yes there are other explanations possible, but you see big bubbles in the average length of careers, average age, and more older players during the years the draft was a complete wreck with HS players, and the subsequent college players who were coming out earlier than before, and competing against lesser competition due to the best talent never making it to campus. Once the Prep to pro's were banned, and all had to spend a year in college (or elsewhere) it somewhat stabilized the draft. Kids are still leaving early as frosh and sophomores, but GMs have more data to go on, and you have "less" busts then you had 15 years ago, and has brought the average age of the NBA back down some, and lowered the number of vets (older players). Note when I talk about "vets" in a negative light, I don't mean anything by it, and I am certainly not talking about the older players who are stars such as Kobe, Dirk, KG, etc. But in an ideal world, older vets who are not bench or role players, would have been replaced by younger guys. Guys like Nazr Muhammad, Vince Carter (oldest player in the league), Turkoglu, John Salmons, Brendon Haywood, Elton Brand, Joel Anthony, Mike Miller, Francisco Garcia etc, those guys should have been washed out of the league by now and replaced by guys who were drafted 5-6 years ago who are still around. Adding another year between HS and the draft helps the scouts even more, helps the players too (you can develop in the NBA, but you develop faster in college), and would help the long term health of the league. But at the expense of current veteran jobs. At least IMO.