I read through the story again and could find no mention of Shuga being a freshman (19); as far as I could see, it's only said that he's a university student. He could just as well be a sophomore at 20 years old, considering Akane's birth during the time they were apart. Now, I could've missed the mention of Shuga's grade, and if so, I'm in the wrong, but if his age/grade is indeed left ambiguous, I think it's a bit weird that some people are so insistent on not only making him barely legal, but in doing so also insist that Oto was knowingly dating a minor in the past (because Shuga would've had to have been a minor (16-17 at most) in the past if he was 18 now). Yes, Oto acknowledges that he dated a high schooler, but he never once acknowledges to having dated a minor.
Some people keep claiming that Shuga is 18 years old now, but not only has that not been established, that's also highly unlikely. He was in high school when he and Oto met and is now in university, but during the time they were apart, Oto went through an entire pregnancy and raised Akane to a point where he's able to be left at daycare for an entire workday (which usually has a 9-month restriction). Akane is on the verge of saying his first words, which is normally what babies do when they're around 12 months of age.
They've been apart for almost 2 whole years. Considering Japanese school ages, even if Shuga is a freshman at uni, he'd be well past his 19th birthday; depending on how far into the year he is, he could just as well be 20. If he was actually 18 now, he would still be in high school (since Japanese students graduate from high school at 18). He might have been 18 when they met (last year of high school), and after almost two years, is about to become a sophomore at uni. Either way, no, he's not 18 years old.