Can't remember if it was mentioned, possibly in an exra, I know there is one where they really met for the first time, and that was way before. The problem with this one is, that there is the manga and the novel, it has both iirc. He has two younger siblings, the youngest is 23. Someone is translating the novel of his youngest brother right know and there the age gap goes in a different direction, the omega is older there. My guess off the seme's age would be mid twenties - late twenties. His omega firend Iraya just made his master and I think they are considered the same age.
The biggest problem with this one is the art, it is beautiful yes, but it makes the omega look younger than he is and the seme most likely older than he is.
Riku is 17 and since he is making a debut in the society, he is most likely already see as someone open for marriage (i know he still goes to school). In the first chapter he looks more mature, but in later chapters they made him look much younger. Compare him to his childhood friend who has the same age, he looks much older. And the seme, the problem is the hair color ( I think it's ash-brown in his brother's novel) and the overall look to make him look like an ice prince. It makes him look older than he most likely is.
How old is the alpha again?