My understanding is that these relationships were seen as educational, between a mentor and a student, and that when the younger partner came of age, any previous sexual dimension to the relationship ceased.
From my 21st century perspective, I have a hard time imagining being in a, say, five year sexual relationship with someone, and then suddenly stopping just because I got a ritual haircut. (I know this is an extreme simplification, but I'm having trouble wrapping my mind around it.)
Do we have any primary sources that address the emotions involved in the transition from mentor/mentee, lover/beloved to equal men? Would an erastes have been sad to lose his eromenos, or proud that he had become a man? Would a wakashu have felt lonely after his nenja stopped writing love letters?
It's worth noting that in Greece, at least, it didn't always stop. Plato's Symposium features a couple that continued to be lovers even after the eromenos (the younger partner, who was penetrated) came of age.
My classics professor said that eromenos who did this would be talked about behind their backs and generally chuckled at, as this was, indeed, a deviant behavior. Sex in ancient Greece was understood not in terms of gender (gay, straight, bi, etc.) like our modern conception of sex, but in terms of penetration (the dominant partner penetrates, the submitting partner is penetrated. There is also a fascinating distinction between familial love and erotic love, and it was considered obscene and inappropriate for your wife to be in the latter category. But that's a separate topic. Ancient Greek sexuality is fascinating!). And it was considered emasculating for a man who had come of age to continue to submit to penetration.
However, it did happen. Like you said, it can be hard to be in a five year relationship that means something to you, and then abruptly stop it one day because society expects that of you. Greece was no less a stranger to forbidden love than we are today.
As an additional question:
How would these relationships start? Would there be a sort of match making service via a third party or through the parents of the eromenos/wakashu?
Not an answer, but a slight correction. While your "wakashū" with the long ū is technically a correct pronunciation for 若衆, it's far more common and acceptable to spell it "wakashu" with a short "u". FYI.
What would be the age difference between these two men?