Year	Age
1970	3.4
1971	3.6
1972	3.8
1973	3.9
1974	4.1
1975	4.2
1976	4.4
1977	4.6
1978	4.7
1979	4.9
1980	5
1981	5.2
1982	5.3
1983	5.5
1984	5.6
1985	5.8
1986	5.9
1987	6.1
1988	6.2
1989	6.3
1990	6.5
1991	6.6
1992	6.8
1993	6.9
1994	7
1995	7.2
1996	7.3
1997	7.5
1998	7.6
1999	7.8
2000	7.9
2001	8.1
2002	8.2
2003	8.4
2004	8.5
2005	8.7
2006	8.8
2007	9
2008	9.1
2009	9.2