Assuming the Translator's note is accurate and Yuuri's studying for his high school entrance exams, he'd probably be around 15. I had a hunch that Wakame Konbu was drawing him younger-looking than his "actual" age.
@Ozonosfera That's almost certainly the case but I think he'd be intending on going to an elite high school (I'll just assume that the parents already paid for it somehow before they died) rather than a normal one where the Grade 10 course content is probably a year or two behind what he was already studying.
@adevaleev What?
She was 40ish at the time. I'll be turning 55 this year, she does not want to be reminded that she'll be turning 70. But she doesn't turn a hair at walking 14K.
Our daughter is turning 24.
On second thought, the translator's note about the book likely being a study guide for high school entrance exams might be inaccurate. I forgot that Yuuri's elite private school is explicitly mentioned in chapter 6 of the serialization to be an elementary school, Adele Elementary. I suspect he's in grade 6, which would make him around 12.
It also could be that the school is so advanced that the students have the option of skipping junior high entirely (not sure if that's realistic for Japan, then again, the country in which the manga even takes place is ambiguous) so that the translator's note is correct and Yuuri is trying to get into senior high as a child prodigy. Or, perhaps, the junior high school that Yuuri wants to get into, possibly on scholarship, has an entrance exam of its own.
@SasazukaIchimashi I just went and looked up books on amazon JP with the name "wakaranai tokoro" and all I could find were books for high school entrance exams, full titles being "高校入試のわからないところ検索ブック" with "高校入試" meaning "Senior High School entrance exam"
I went back to that serialized chapter to make sure I hadn't made a mistake translating "elementary school" and it really is that. So, unless "wakaranai tokoro" refers to something else, it does look like Yuuri might be skipping junior high.
@Zorikinu I can't think of much else that would make sense given the context. While we don't know any specifics about Yuuri's academic performance, we can only assume that he's a high achiever due to how seriously he takes absolutely everything in his life and how he even dresses semi-formal at home.
Also, the sooner he's out of high school, the sooner he can marry Lilith, I suppose.