I am suprised this thread got bumped after 2 long years.
UPSC being a constitutional body, is fully sovereign to regulate its own procedure. As mentioned by the OP, there are 2 OM regarding/regulating this where it is stated that in exams which are conducted in two phases, and where the mains happen in the second half of the year, the age will be reconed from August of that particular year.
Irrational and Unfair- Absolutely. However, it will be extremely difficult to challenge it in a court of law (as the body in question is a constitutional one). Our only hope- if the government itself amends the provisions.
