@phoenixir I think he was around 9 when he met the twins, It was probably supposed to be just "a few years have passed", I don't know for sure but following the timeline should be around that (I think Biamam Scans somehow made a mistake saying he was 12), then a few weeks later (probably less than a month, or just a little over a month) they did the silvermoon vow. After that there was a one year time skip (not sure if It was exactly one year or "a little more than one year", translations are deceiving), then a few weeks (probably a couple weeks or so, again, I'm not sure) after the picnic, Erina's health worsened and half a month later she died, after that I think a few months passed (I'm guessing It was at least couple months), since they didn't seem to be so sad about her death anymore when they went to do the school examination test. Around 11 years old they entered the school, and one year later, when they entered the second year of school, It was said that Sirius had turned 12. They are all around the same age as far as I know, just a few months apart from each other, and It was stated that Reese is older than Sirius when she became his apprentice, but he is older than the twins, and Reus is the younger one of the twins.
Anyways, that's just what I think, but that's what makes the most sense for me, considering the things that happened and the things that have been said, so I believe that the "I turned 12" from before he met Emilia and Reus was a bizarre mistranslation. I could be wrong, but I don't think the author himself would make such a mistake, so I'm betting the issue was on the translation's part. Either was Biaman Scans that made a mistake back on chapter 2, or Champion Scans made a mistake here, saying he turned 12 now. But I'm fairly sure the mistake was back on chapter 2.