Ch.1 pg. 11 "And although he has yet to reach ten years of age, he inherited the title of duke"
It could be the words or translation is confusing, and at the same time, it's also mentioned on the same page, "It's no surprise that you were chosen as the most handsome knight in the Unicorn Knights for three consecutive years." Which is another layer of- I don't think this guy is 10, because it would imply that eligible people like older women/men would be voting for his as the 'most handsome,' usually these sorts of rankings confirm attractiveness/marriage eligibility.
Height and pure intent aside, his stoicism and sense of responsibility leaves me confused on how old he is. As we aren't given as straight forward of an answer on his age like her family members where their age is included in the text box.