Year	Age
1970	3.8
1971	4
1972	4.1
1973	4.2
1974	4.4
1975	4.5
1976	4.6
1977	4.8
1978	4.9
1979	5
1980	5.2
1981	5.3
1982	5.4
1983	5.6
1984	5.7
1985	5.8
1986	5.9
1987	6.1
1988	6.2
1989	6.3
1990	6.5
1991	6.6
1992	6.7
1993	6.9
1994	7
1995	7.1
1996	7.3
1997	7.4
1998	7.5
1999	7.6
2000	7.8
2001	7.9
2002	8
2003	8.1
2004	8.3
2005	8.4
2006	8.5
2007	8.6
2008	8.7
2009	8.9