Ryuuga is an android and literally ageless so the Ryuuga-student relationship looked much worse when she was young than it does in the end. Unless he is killed, he is going to outlive her and not look a day older than the day he was created. I'm more wondering what happened to the family he had before he got formally involved with his student. We only see them briefly, but we see them. He has a wife and a kid (undoubtedly from a relationship she had before Ryuuga, the kid is too old to be his even if his is "fully functional"). Somewhere between then and now his relationship with her must end and a new one with his student began. We aren't given any information about when this happened. So it might not have even happened until she was already an adult. All we know is that there was some sort of attraction between the two while she was still a student, but there is nothing in the series to say that anything ever happened between them while she was still in school.
I've always been of the assumption that the infant Mahoro was a clone of the real Mahoro, the Saint scientist, with the memories of the other Mahoro, the android, implanted into her via Matthew, who retains all of android Mahoro's memories, as it is clearly stated that android Mahoro was an extension of Matthew. Assuming that clone Mahoro lived just as long as the fake age that android Mahoro was given (19) before "reuniting" with Suguru and knowing that Suguru was 14 during the entire series, then that would make adult Suguru 33 and clone Mahoro is 19. I, personally, would have preferred that she would have had her aging artificially accelerated to decrease the age gap, but it doesn't really bother me, even if, under 2004 law, Mahoro would still be considered a minor until she turned 20.
Gainax ruins everything. I try to pretend the animé ending never happened. Also, the Chobits animé has almost 10x as many user entries on MAL as Mahoromatic (240K to 25K). Even Love Hina only has 130K entries on MAL and Love Hina was everywhere in the early 2000's. Don't kid yourself, Chobits was as mainstream as mainstream could get when it was released in 2002.