She is about like...60 years or so in human years.
I don't really see anything indicating her age, other than that of her looks as stated by ZUN, that her appearance is of a girl in her early teens (10~14). As for her actual age, there was a very good description of her age once in a fanwork, which stated that she has existed ever since Humans Have Feared Darkness Which could mean as far back as cavemen times, as early non-nocturnal human may have ventured carelessly into the darkness of night, only to be chomped down by a dire wolf or saber tooth tiger, instilling a fear of the dark unknown in the hearts of mankind. Hence, BAM! A youkai is born.
