How old would I be if I graduated in 1988?

As long as you passed every grade and started school when you were 5 then:

1985 + 5 = 1990/1991 (Starting Kindergarten)

1990 + 13 = 2003/2004 (Graduating High School)

I felt the need to tell you that you gave the best answer, this way as long as someone knows math, they now have the formula to figure out for others as well:) ahhhh knowledge isn't it great?