I think it was stated somewhere that anyone who messes around with magic is bound to become a youkai eventually. Marisa was trained by a youkai, lives around youkai, and all her non-miko friends are youkai, so I would say the outcome there will be pretty obvious. It's just a question of how long it takes. (I strongly doubt the series will advance far enough chronologically to see it, but yes seeing Marisa hit boobinite tier would really be something.)
Anyway, only the humans are subject to aging, and offhand I think there are like, maybe four pure humans? Reimu, Marisa, Sakuya, and Akyu. Marisa will be the demons sooner or later, Sakuya's origin is still up in the air (though she insists she is a normal human, she should be aging very quickly due to spending so much extra time in a day compared to everyone else, doesn't seem to be the case), and Akyu won't live long enough to see old age anyway.
As for the half-breeds:
Alice - was a human, so she aged from her PC-98 incarnation, but is a youkai now.
Sanae - She's aged normally up til now but her half-god side is presenting itself more and it might stop her from aging.
Rinnosuke - Confirmed to be aging slowly
Keine - Age never stated, but probably going at a rate similar to Rinnosuke (if not slower, she might be very ancient, given her status).
Youmu - Human side seems to be aging normally, but Youki is at least 300 years old, so at the very least her life(death?)span is extended.
Mokou - hurr
So really, the only one that's guaranteed to get significantly older looking is, ironically, Reimu.