Several months ago I saw a math birthday t-shirt on the computer gear
website, but it seems to be no longer available from them.  The shirt
says "age=" and then depicts a complex mathematical expression that
evaluates to 60.

Need to find this in order to get it hear by new-year's day (2006). 
If this cannot be located, a complex mathematical expression that
evaluates to 60 would be good.

The expression could be arbitrarily complex. Here is a simple one:

5!/2 = 60

You can make this more complicated using the gamma function (replace
the G in the following with the upper case Greek letter gamma):

G(6)/2 = 5!/2

You can get really complicated by writing the gamma function as
defined in terms of an integral. See equation (3) at the following
link.

http://mathworld.wolfram.com/GammaFunction.html

If you set z=6 in equation 3 and eliminate the factor of two, it
evaluates to 60. Here it is written imperfectly in text:

I(0 to infinity) exp(-t^2)*t^11*dt, where I is the integral sign

If you would prefer an infinite sum to an integral, you could use the fact that:

zeta(4) = pi^4/90, where zeta is the Riemann Zeta function, which can
be written as an infinite sum as in equation 7 at this site:

http://mathworld.wolfram.com/RiemannZetaFunction.html

A little manipulation gives:

60 = 2*pi^4/(3*zeta(4))

Or choose your favorite mathematical function and work from there. For
good measure, you can always through in a factor of exp(i*2pi), which
is equal to unity.