Beside the rear lens mount of many Canon lenses is a date code in the form of "UT0805".  The first letter, "U", indicates that the lens was made in Canon's Utsunomiya, Japan factory.  The second letter, "T", is a year code that indicates the year of manufacture. Canon increments this letter each year starting with A in 1986 and prior to that, A in 1960 without the leading factory code. The first two numbers "08" indicates the month that this lens was manufactured in. Month 08 is August, month 11 is November. . The next two number "05", are meaningless in determining how old a Canon lens is. This is a Canon internal code. Here is an example and a list to make things simple:

Example: UT0805

U= Utsunomiya, Japan factory; T=2005; 08=August; 05= Canon internal code meanigless

A = 1986, 1960
B = 1987, 1961
C = 1988, 1962
D = 1989, 1963
E = 1990, 1964
F = 1991, 1965
G = 1992, 1966
H = 1993, 1967
I = 1994, 1968
J = 1995, 1969
K = 1996, 1970
L = 1997, 1971
M = 1998, 1972
N = 1999, 1973
O = 2000, 1974
P = 2001, 1975
Q = 2002, 1976
R = 2003, 1977
S = 2004, 1978
T = 2005, 1979
U = 2006, 1980
V = 2007, 1981
W = 2008, 1982
X = 2009, 1983
Y = 2010, 1984
Z = 2011, 1985
................

PS. Lens date code indicates the date of a lens manufactured, but it does not necessarily mean the years of lens usage, it is not uncommon for a lens sitting in a retail store for 1-2 years.