IMO around 17
I think more or less the same, Reed, Ben and Doom should be past 40s since has been like 20 years after they finished the university, Johnny maybe is on his last 20s and perhaps Susan on his last 30s.
Back to Kristoff, my head canon is that he was adopted at 5-6 years lived a couple years with Doom and then happened the brainwash incident at 7-8, to complement the wonderful information that you provided let me add to the fact that Kristoff is regarded as being originally "few years older than Franklin"
And there is this page when he makes his first magical grow up and suddenly becomes a 12 years old while Franklin remain the same.
My idea of Kristoff being around 17 comes from this days. When he was shipped with Cass Lang and the age gap was stated to be just a year. Since she was labeled at 16 at the moment of her resurrection, I think that he should still be a year older making him 17.
Yes, don't seems that he will get a participation soon. Perhaps we should opt for the letter, our demands are fair, we don't want him getting his own run, but at least, acknowledge his existence people.
I think that part of his tragedy comes from Doom need to show off and inability to accept imperfection. Kristoff was more or less happy the first years, but after the brainwashing incident, Doom becomes much more cold and distant and sometimes plainly cruel (he got Kristoff dead/in comma in one of his schemes against Richard). My head cannon is that Doom being a perfectionist can't accept that failed as a parent (getting the boy he was supposed to protect badly hurt) and instead of just accepting his guilt and try to become a better parent, he got enraged about being unable to top Richards. Richards is actually a flawed but more or less effective parent who despite his mistakes and ignorance kept trying.
So Doom try to regard Kristoff as a failed experiment and pretends to use Valeria to show his "superiority", what better way to show that Reed is a second rate paret that getting the love of her own daughter? What he can't see is that even whe Valeria loves him, he is still a kind of "cool uncle" nor a parent, while Kristoff is his actual son and still in need of guidance, so in his pursuit of show off against Reed he is neglecting his own son.
That makes still more frustrating the whole Secret wars issue. Because just when I thought that Doom was about to make a real effort to rise and guide Kristoff, the boy gets dropped again in order to take care of the Richards family.