@tonicwalter The state of the mind has nothing to do with the state of the body. From your point of view, Shin wouldn't be able to be with anybody at all. He wouldn't be able to be with a 15 years old because he has lived for more than twice that time (his soul was in his twenties and he lived for 15 more years) and he wouldn't be able to be with someone with his spiritual age because that would be the same case of disturbance, but in reverse.
If that isn't disgusting, I don't know what is.
This suggest that you don't know what should be considered disgusting.
Have you ever met a teen around its 15 who's more mature than adults in their 30's? I know I did because I was myself more mature than most adults, during my time as a teenager. That's basically what's happening with Shin in this story. His "past" only give him more maturity and mind openness than other kids his "new" age. There's also the point where living in a body for many years usually makes you forget about your past life unless something requires you to remember, making someone like Shin more of a 15 years old boy than his actual age. (This is a well known psychological effect seen investigation where any undercover agents must undergo an physical and psychiatric evaluation and, in most case, have some assistance as they were doing a role for so long that, sometime, they forget who they truly are.)