I think that has never been asked. I mean from what I have read so far. I think the factors are,
age
whether highest level of education is relevant to flight. (physics yes, mech engineering yes, biomedical sciences no, business no.)
Results of science subjects in O and A levels.
eyesight (this one i was told by my sis-in-law's bro that it's very important. he was maintenance engineer with SIAEC)