the author making uke 17 actually has a reasoning in my eyes cause the story progresses from the fact he is getting bullied and to be theoratically clear with you high school is more common for bullying than college. The reason why author didnt make him 18 is because of time skips probably, if he meets shi liu at 17 and gets abducted during that time frame he will eventually age up to 18 and during that time the bullying will still take place and it probably is the age group he is in right now in this chapter. And i agree with you on the fact its not really grooming and i think it was the authors intention to make the uke look vulnerable but the options to how and why he would look vulnerable are very low and making him a highschooler is the most common and reasonable choice due to the reasons of him being more vulnerable during that time and state would make more sense than him getting bullied in/at college. And him looking young is probably due to authors desire to add 'realism' in his malnourishment lol.. but if you really put together the age issue it makes sense when the story progresses, not saying its right but it adds more tension to the plot of 'unavoidable circumstances'. 

this is just my theory! feel  free to tell me your own opinion on this take