One mathematician in front of an equation-covered chalkboard explains to his friend that he is “too old for this shit,” claiming that math seems to be getting harder as he gets older. The punchline is that it turns out that the mathematician is only 13, which is funny because 13 hardly seems old at all.
This strip is almost certainly a reference to the last decade of mathematical breakthroughs found by very young students such as Sarrah Flannery, who in 1999 published a ground-breaking mathematics paper at the age of 16.