A lot of people have already commented about this.
Since modern romance often ties "love" to physical desire, people tend to forget that in literature (considering this manhwa is an adaptation of a novel with the same title), first love can also be about pure admiration, appreciating someone's beauty, or even a meaningful moment that leaves a deep impression without any sexual desire at all. And that's exactly what we see here. It's still valid to call it first love. You just need to understand that love can take many different forms, so don't apply your own understanding of love to what Yuri meant when he called it his "first love," because that's obviously going to make you feel icky. If you still find it weird because Yuri was 25 while Xinlu was 12, that's exactly why POV matters. We get to see how Yuri himself describes those feelings, and based on what's actually shown in the story, they don't fit the medical criteria or legal definition of pedophilia either.
I like your response its definitely a matter of perspective when it comes on to identifying something or someone as a first love, doesn't necessarily have to do anything with sex or romantic attraction all the time, this story definitely showed both sides of the same coin when it came on to that, even the last chapter where xinlu was talking abt what love was supposed to be, and gable did share is way of feeling or understand love in his own way without judging xinlu of his intense feelings or ways of displaying them. Either way I love this story of them both, cuz as gable said love can be many things.

i'm at ch three and i wonder if the drowning boy and the one saving him are going to be a couple if so that's really disturbing