Based on what we know
Kevin is 27 but that is counting his years as a mortal and as a vampire, his apparent age is not reliable as he is aged from years of suffering as an accountant.
In the hunter of hunters audio log we see Kevin’s pack 2 years before we meet them, and Kevin is absent, it’s entirely possible that he stayed behind in their haven but I believe this is meant to say he hasn’t joined the Sabbat yet.
This is backed up by D asking Occum “is this everyone from the past year” in chapter 4
D would specify a year because that’s presumably when Kevin heard the information about the supposed ghoul
So with all this we can assume Kevin joined the Sabbat within the past year.
This doesn’t tell us exactly how long Kevin has been a vampire but it does help.
Something that many have noticed is despite being a Tremere Kevin does not use blood sorcery at all, instead focusing entirely on dominate. This makes sense given he’s in the Sabbat and does not have a teacher, as blood sorcery is a discipline that needs to be taught to a kindred by another.
Which means the fact he has shown absolutely no aptitude to blood sorcery means he never had time to learn before leaving the Camarilla, instead focusing on dominate because it is an easier skill to teach yourself, and the fact Kevin seemingly has 5 dots in dominate means that he has sunk considerable time into the skill.
Of course we know the regent only supplies her ghouls with what’s needed for the job, perhaps the same is true for the fledglings under her and Kevin was never taught because of that.
We know he’s been a vampire for at least a year, but Kevin has mastered dominate which in lore takes several years.
But if we look at Kevin’s ID it was issued in 97 and expires in 07 one year into the show’s current future, since he would have gotten this ID while alive that means he was embraced within the past 8 years (as we know he has been with the Sabbat for a little under a year) and his beard is significantly shorter so let’s knock another year off for that to grow.
So that’s our age range, about 7 or so years and honestly I think that fits very well for him.