Year	Age
1970	3.7
1971	3.9
1972	4
1973	4.2
1974	4.3
1975	4.5
1976	4.6
1977	4.8
1978	4.9
1979	5.1
1980	5.2
1981	5.4
1982	5.5
1983	5.7
1984	5.8
1985	6
1986	6.1
1987	6.3
1988	6.4
1989	6.6
1990	6.7
1991	6.9
1992	7
1993	7.2
1994	7.3
1995	7.5
1996	7.6
1997	7.8
1998	7.9
1999	8.1
2000	8.2
2001	8.4
2002	8.5
2003	8.7
2004	8.8
2005	8.9
2006	9.1
2007	9.2
2008	9.4
2009	9.5