If Nick was born in 1982, and came back from Savitar's Island in 2006 ( a little under two years after his death) then he was 23 or 24 when he became a dark hunter. So how could he be 26 (according to Vane) in Night Play and still human?
I try to make the books as realistic as possible, and especially the people in them. In real life, we all do that with ages. We round up or or down, or guess as to what we think they are. Case in point, I keep saying my son is 20, even though his birthday isn't until next April. I know he's really 19, but I say 20 to remind myself that he's a grown man and that I shouldn't be wiping his chin in public these days :) I also have a girlfriend who is two years younger than me, but for some reason, I can't keep that straight and am always telling other people she's older than she is and it drives her nuts. Vane knew Nick was in his mid 20's and said the age he thought was right.