I´m 39, 6´2" and weigh a bit over 100 kgs. First thing i did after getting my PPL licence was a Basic Aerobatics course, and am now training for my first competition (in the "Sportsman" category, on an S2B).
I´ve done -3 and +6 Gs, and have had no problems whatsoever, though being on the tall side i´d supposedly do less well in sustained high G turns. Don´t know. Done 270 degrees competition turns pulling medium Gs with no problem at all.
I´m currently miserably unfit, btw.
My cholesterol levels aren´t the best around, wich i gess helps prevent blood escaping towards the feet in a hurry come "high" +G maneuvers (not that i´m recommending raising cholesterol levels here, mind! I´d swap mine for lower ones right now
).
Anyway, if i had to gess, i´d say low blood pressure is not good for aerobatics (at least not for high +G), but i´m very far from being an expert, so there
I´ve read some interesting stuff on the subject on some issue of, i think, the "Journal of the British Aerobatic Association", wich you can find online (haven´t got the link here, sorry, but i´m sure you´ll find it easyly).
I understand that the best exercise you can do to prepare for high G is weight-lifting, as has been said here before. So i mainly do pint lifting these days
(just kidding...sort of...)