Answer
$53$
Work Step by Step
The prime numbers less than 100 are:
$\begin{array}{ccccc}
&2 &3 &5 &7 &11
\\&13 &17 &19 &23 &29
\\&31 &37 &41 &43 &47
\\&53 &59 &61 &67 &71
\\&73 &79 &83 &89 &97
\end{array}$
Notice that the consecutive prime numbers $47, 53, 59$ are the only three consecutive prime numbers that are equally spaced.
Thus, the middle-aged man must be $53$ years old.
