Base on what I can understand it's 10 years gap tho. They put a hint to the panel where Izumo explaining to ryuuta about how grateful he was that Ryuuta taking care of him 10 years ago.
i think theyre around the same age since they both looked young in the flashback when they were kids
That just means that they meet 10 years ago, not that there's a 10 year gap between them. Izumo is just a few years older than Ryuuta, in his early twenties. At the beginning of the story Ryuuta is 16.
Praying that the age gap is only 1 to 2 years