It's apparently 13 in Japan and controversial AF globally. But more practically apparently 16-18: '[...] many prefectures also have local "corruption of minors" or "obscenity statutes" (淫行条例) which raise the de-facto age of consent to 16-18, unless they are in a "sincere romantic relationship", usually determined by parental consent. For example, the effective age of consent in Tokyo by local statute is 18. The age of marriage is 16 for girls and 18 for boys with parental permission, and 20 otherwise (as stated in " ナス邃「窶慊カ窶「ナクナスニ停?邸", the Child Welfare Act of Japan.' https://www.ageofconsent.net/world/japan
A 10 year age gap when they're both 'adults' is a completely different thing. A romantic relationship between a child and adult isn't consensual- the gap in intellectual experience, ability and reasoning is far too big at that stage in my opinion.
Am I enjoying the sexual liberation feel of the manga- yess! I just wish the protagonist was an adult. The age gap would be meh if she was 18 and above 😞