I want to describe how I arrived at the numbers used for average number of citations for papers in mathematics.
The starting point was the data given on The Times Higher Education Supplement. You can find these on the web if you search for "Average citation rates by field 1998--2008" These data are also given below.

Year-Citations
--------------
2008-0.11
2007-0.55
2006-1.27
2005-2.09
2004-2.83
2003-3.44
2002-4.12
2001-4.38
2000-5.03
1999-5.65
1998-5.73

What we need to do next is to introduce an age for the average paper. It is good to assume that the average paper is published mid year. Now we can extend the above data by adding age of paper in years.

We obtain

Year-Age-Citations
------------------
2008-00.5-0.11
2007-01.5-0.55
2006-02.5-1.27
2005-03.5-2.09
2004-04.5-2.83
2003-05.5-3.44
2002-06.5-4.12
2001-07.5-4.38
2000-08.5-5.03
1999-09.5-5.65
1998-10.5-5.73

Now we define

x = age of a paper (years)

y = number of citations

The claim now is that the above data are reproduced very well by the following linear model.

Linear model: y = 0.59682 x - 0.0825

Consider R^2 in [0, 1] which is a standard measure of the predictive value of the model.

For the above mofdel

R^2 = 0.987 - very high

Note that the age of an average 2008 paper is put at 0.5 years. This helps bring the constant in the linear model close to zero (ideally the constant in the linear model would be zero, since a paper of zero age should have zero citations).

Points generated by linear model including extrapolation:

Year-Age-Citations
------------------

2008-00.5-00.21
2007-01.5-00.81
2006-02.5-01.40
2005-03.5-02.00
2004-04.5-02.60
2003-05.5-03.20
2002-06.5-03.79
2001-07.5-04.39
2000-08.5-04.99
1999-09.5-05.58
1998-10.5-06.18
1997-11.5-06.78
1996-12.5-07.37
1995-13.5-07.97
1994-14.5-08.57
1993-15.5-09.16
1992-16.5-09.76
1991-17.5-10.36
1990-18.5-10.95
1989-19.5-11.55
1988-20.5-12.15
1987-21.5-12.74

The final step was to assume translation invariance which is a very good assumption given that the model is very linear, so that we can say that, from the viewpoint of someone in 2010, the average citations will be the same as those above, but shifted by one year forward. This gives

Year-Age-Citations
------------------

2009-00.5-00.21
2008-01.5-00.81
2007-02.5-01.40
2006-03.5-02.00
2005-04.5-02.60
2004-05.5-03.20
2003-06.5-03.79
2002-07.5-04.39
2001-08.5-04.99
2000-09.5-05.58
1999-10.5-06.18
1998-11.5-06.78
1997-12.5-07.37
1996-13.5-07.97
1995-14.5-08.57
1994-15.5-09.16
1993-16.5-09.76
1992-17.5-10.36
1991-18.5-10.95

The above are the numbers that I used in my analysis