Happy Birthday, Chihaya! How old are you turning? ...haha, video game characters don't age.
But I've done some calculations and concluded that, technically speaking, you're 24.
Why 24? Easy. The first game came out in 2005 when Chihaya was 15. She's existed for nine years since then. 9+15=24. Therefore, Chihaya is 24. Officially speaking.