I realize I wasn't getting notifications from the forums here, so I missed this. Sorry!

As for your question, the kobolds maturing at a faster rate is intentional, but that is an unfortunate side effect of doing that. I made them mature faster so children born in your fort won't be idle and getting into trouble for as long (which would've been a bigger problem for these kobolds since they have a lot of kids at a time) and because I liked seeing kobolds born in the fort become productive members of the fort before I suffer FPS death or something.
If DF gave more levers to mess with when it came to a creature's age or maturity, I would've pushed them finding love out to an older age.

If you want to modify it, you can go into the "creature_kobolds.txt" file, and CTRL+F for the tags "[BABY:1]" and "[CHILD:4]" - the number indicates the age which they mature past that stage, so kobolds become children at 1 and adults at 4. Just keep in mind it will revert if I update the mod!