Aya would be more interesting if she were a ninja, but sadly she's just a speed freak crow tengu that likes making a boring newspaper.
The only ages we have any real idea on are Flan who is at least 495 (how long she's been in the basement), Chen (she has to be at least 10 because that's when a cat can grow a second tail and become a youkai), Ran (with 9 tails she's at least 800), Mokou (she's been immortal for something like 1,300 years) Reisen (she fled the war between the moon and the US in 1969), Maribel and Renko (both college age), Kogasa and any other youkai born from objects (an item comes alive and becomes a youkai after 100 years), Youmu's grandfather Youki (at least 300), Hijiri, Shou, Ichirin and Murasa (all at least several centuries old), Patchouli (she's been in that library for at least 100 years), Reimu, Marisa and Sanae (who are in their teens)... and then it gets all kinds of murky, except for Yukari, who is ALWAYS 17.
Utsuho was modified by Kanako, who basically allowed her to consume yet another god. Her body changed to control nuclear fusion, which was a ploy by Kanako to provide Genskoyo with nuclear power... mainly to gather faith (another Moriya Shrine conspiracy!). Utsuho was picked because, well, she's dumb, as the dumber the person who controls the power, the better they can use it... because why not.