I guess that the seme is in his mid/late twenties.
Iraya and him should be around the same age (childhood friends), and Iraya just made his master degree. So i think they are still in their twenties. He has two younger brothers, the youngest was 23 when Keiki searched for his fated pair by hosting those parties where he met Riku. (the last part refers to some reviews of readers who read the lightnovel about his youngest borther.)

Okay, I know that the uke (forgot his name cause I've been neglecting my ongoing list for a while) is a highschooler, 17 if I remember correctly BUT DAYUM DANG HOW OLD IS THE SEME? He looks like he is something between 35-45 and I am not sure if I dig it? Isn't this kinda grooming? The art style is really enjoyable although the story is pissing me off currently. I hope we find out more about the uke's original family and see the seme being more kind/understanding towards his soulmate. Also, I don't like Iriya one bit rn, all of them are being too hard on the MC.