Among the technical difficulties, the key element is the jumping ability. It is easier to defy gravity when you are small in statue and have better spring (limber body), both of which are the hallmark of youngsters. As female skaters hit puberty, the center of gravity is lowered due to the enbroadened hip area. Also the fat/muscle ratio changes (increase in fat). Among others, those two factors make it harder for female skaters to continue defying gravity, i.e., jumping.
This is why many late teen and adult female skaters watch out their diet like a hawk and stay slim and muscular - the nature of the sport gives advantage to such physical types. That such an already excellent jumper like Miki felt need to lose significant weight during the summer is a good example. I also wonder whether she also lost weight to attempt quad, again - something she did when she was younger but not recently though I heard the rumor.
I do not know the point at which the technical difficulty becomes unrealistic for female skaters. Also any human endeavor has corrective mechanisms. When the technical difficulty reaches a critical mass, e.g., too many splat-fests resulting from skaters attempting arduous jumps, the skating federation will enact new rules. These "improvement/changes" are incremental and continuous.
One other factor to be taken into account is the human potentials. We truly do not know the limit as to what human body is capable of. I remember the days when it was considered impossible to break 10 seconds among 100 meter sprinters. We have way passed 10 seconds and still counting.
Same thing goes with the progression of jumps in skating from a single jump to a quad. Are we on the verge of seeing "penta-jump?" I don't know. But I would not count is as an impossibility.
Pushing boundaries like that goes hand in hand with new knowledge and new understandings. For instance, tennis was predominantly played on glass and clay surfaces initially. As the hard court becomes popular, players discovered how "hard" a hard court was to their knees. Manufacturers in turn have continue improving better shoes to absorb constant pounding, thereby help extending player's longevity. I suspect the similar improvement is occurring with skating gears (help me out here.)
Anyway, those are some of my thoughts. Feel free to add your wisdom here.