I’m not sure what Jack thinks is the ‘clear and correct’ answer, as he didn’t say.
This is a matter of social convention, not calculation. No-one was actually counting decades after the beginning of the Common Era (the supposed year of the birth of Christ according to (probably erroneous) mediaeval scholars) in the years immediately after. Therefore there is no continuity here and no reason why we can’t regard the first decade of the Common Era as having only 9 years, or extending from 1BCE to 9AD.
A decade is generally regarded as a grouping of years having the same three initial digits. Therefore the new decade has begun.