Year	Age
1970	3.9
1971	4.1
1972	4.2
1973	4.3
1974	4.5
1975	4.6
1976	4.7
1977	4.9
1978	5
1979	5.2
1980	5.3
1981	5.4
1982	5.6
1983	5.7
1984	5.8
1985	6
1986	6.1
1987	6.2
1988	6.3
1989	6.4
1990	6.5
1991	6.7
1992	6.8
1993	6.9
1994	7
1995	7.1
1996	7.2
1997	7.3
1998	7.4
1999	7.5
2000	7.6
2001	7.7
2002	7.8
2003	7.9
2004	8
2005	8.1
2006	8.1
2007	8.2
2008	8.3
2009	8.4