According to an editor at JP wiki, the
Oct 2013 issue of
Aera Magazine (published by Asahi Shimbun) had a profile on Kanno which said "Born 1963 in Miyagi Prefecture" (just the year, no day). He also notes that in the 1974 Yamaha Junior Concert, which she participated in as a child, that
Kanno was 11 years-old at the time. These sources seem much more reliable than the other ones, so I'm setting her birth year to 1963.
Edit: For anyone wondering about Kanno being linked to Gabriela Robin, I was able to officially confirm it through copyright listings. Kanno is the sole rights-holder on songs where Robin wrote the lyrics.