Responses
Probably it was mentioned somewhere but if not, the translator may have decided to give an estimate seeing that Yui's in his 3rd year of high school and when they met up when he was younger, Yui was still in elementary. So 3 years of middle school + 3 years in high school would be about 6 years. Hope that helps a bit.
He's currently in senior year of high school? I thought he was a middle schooler.
How did the translator know that it had been 6 years since they've seen each other? I don't see anything that shows that 6 years had passed.