I did a little thinking, and discovered how old The Boss is. There is a mission in Saints Row: The Third where The Boss and Pierce are in a car together. Pierce turns on the radio to the song What I Got, by Sublime. When the song comes on, The Boss says something along the lines of '"This was my life in high school". What I Got came out in 1996, and the normal age range of high school is between 14-18. That means the boss was born between 1978 - 1981. The events of Saints Row took place in 2006, making The Boss somewhere between 25-28. I know from somewhere that the events of Saints Row 2 happen in 2009, making the boss 28-31 in Saints Row 2. Johnny Gat died in 2011, Making The Boss 30 - 33 during the events of Saints Row: The Third. Assuming that Saints Row: IV and Saints Row: Gat out of Hell take place when they came out (2013 and 2015),That makes The Boss 32-35 in Saints Row: IV, and 34-37 In Saints Row: Gat out of Hell.
But have you noticed that the numbers 31 and 13 are recurring in the Saints Row series?
Assuming that those numbers hold true, I'd assume The Boss is 31 in Saints Row: The Third. Now some math. 2011 - 31 = 1980. So there you go. The Boss was born in 1980 according to my theory. But wait, we need to go deeper. According to my theory, The Boss has a three year age differance between each game. There are three sides on a triangle. The Boss is Illuminati confirmed. No, but seriously, the Saints Row timeline[saintsrow.wikia.com] is jumbled up, so this is still a theory. Anywho, thanks for taking the time out of your life to read my pathetic little theory, and now I bid you adieu.

[EDIT]
Okay, so I learned that in order to be president, you must be 35 or older. So that means The Boss is 35 in Saints Row IV and Saints Row: Gat out of Hell , 33 in Saints Row: The Third , 31 In Saints Row 2 , and 28 in Saints Row .