Year	Age
1970	3.9
1971	4
1972	4.1
1973	4.3
1974	4.4
1975	4.6
1976	4.7
1977	4.9
1978	5
1979	5.2
1980	5.4
1981	5.6
1982	5.8
1983	6
1984	6.2
1985	6.5
1986	6.8
1987	7
1988	7.3
1989	7.6
1990	7.8
1991	8.1
1992	8.3
1993	8.5
1994	8.7
1995	8.9
1996	9.1
1997	9.3
1998	9.5
1999	9.7
2000	9.9
2001	10.1
2002	10.2
2003	10.4
2004	10.5
2005	10.7
2006	10.8
2007	10.9
2008	11.1
2009	11.2