I think someone already answered this here at the first reply under this topic. I think you forgot they are in middle school (junior secondary) so they are definitely not 18 or 19 yet lol. Unless you were talking about them in the future when older in which case yes they are about 18-19 then :) The boys are 15 right now since they are in their last year of middle school. (senior secondary is high school in case you didn't know). this is all a flashback to their middle school days before jian yi gets kidnapped/disappears on the second day of high school and doesn't come back for about 3 years, as explained in the beginning of the story. he hasn't been kidnapped yet within the storyline and the boys speak of high school as if they haven't gone yet. it's also mentioned in future special chaps like the christmas ones, when they are older you can see jian yi having to talk about repeating high school over.
http://www.mangago.zone/home/mangatopic/5262227/
how old are they? That's what i asked myself for my fanfic. So i decide to post here the answer about Jian Yi, Zhan Zheng Xi, Mo Guanshan and He Tian's age.
At the age of seven or six, children begin primary school education in China. Primary and secondary education in China takes 12 years to complete, divided into primary, junior secondary and senior secondary stages. So if they were 7 when they started, and we know they are not in primary since they were once called 'upperclassmen' so, in my opinion they are 17 or 18. Maybe 19 if they are in their last year. IDK, what to you guys think?