Let $ t $ and $ u $ be the tens and units digits of John's age. We are told that $ t+u=5 $ and $ 10u+t-(10t+u)=27 $. Rewriting the second equation as $ 9(u-t)=27 $ and dividing by 9, we have \begin{align*} t+u&=5 \\ t-u&=3.\end{align*} Summing these equations, we find $ 2t=8 $, which implies $ t=4 $. Substituting back into either equation, we find $ u=1 $, so John's father is $ 10t+u=\boxed{41} $ years old.