It was explicited, there is the whole scene where Tei tells Shuraka that children aren't supposed to fight, and that around him he can behave as one. And they met 14 years in the past, so if he is 19, then he would he would have to be 5... I'm not sure he is 24 like the other commenter said, but it's a minimum.
Like, I can understand people admitting to see past it because we’ve all turned off our brains before to read a yaoi/BL, we’ve all been there to some degree… but to be okay with it? To endorse it? To not see a problem with it it’s bc either they are children, their frontal lobe isn’t developed, they are ignorant or they are degenerates.

“There was an awkward air about the way he grew so aroused from mere kissing, like an adolescent boy.”
Bro. Maybe because he is? I don’t remember Shuraka’s exact age at this point (because I’m honestly too lazy to go back) I think his 19? Or around that age which doesn’t make it better. I hate how it’s emphasized the difference in age, and how it wasn’t explicitly shown that Tei views Shuraka as more than a child until he touches his dick. And yes, I’m giving it a chance, but still my point/opinion from a few chapters ago still stands.