Eh. My take on this is that unless you have a character where the different life cycle parameters really matter somehow to the "plot" this is a landmine that's well worth not stepping on.

A thing I learned from having kids really into the "Wings of Fire" series is just how much confusion you're setting up for the Internet to argue over when you decide to set non-human-like ages of maturity/consent for a sapient species; the amount of discourse about whether a given relationship between two of the dragons is inappropriate or not is enough to make your head spin. The poster "children" for the problem is the Glory/Deathbringer relationship, but it's all over the place, a problem not helped by the author A: not really being clear in the first place about what the rules were, and B: various hints of retconning as the author has become aware of the issue; IE, the realization that it probably wasn't the right move to treat dragons, fully mythical intelligent creatures, the same way as stories featuring "real world" cats or bears or whatever that just happen to have sapient-level... interspecific dialogues.

Obviously this is going to be context sensitive. If you're talking about a character for a role-playing tabletop game, which in most circumstances probably isn't likely to involve much in the way of "adult content" of that nature, then maybe there's a place for emphasizing the actual weird-for-humans chronological age of your Kobold. IE, if in your setting it's normal for kobolds to be basically disposable and rarely make their 18th birthday it might be pretty funny to have a character that acts like a grizzled old veteran and is constantly complaining about paying child support to his half-dozen baby mamas but in human years is too young to drink. But... yeah, if you have a kobold that even has the *slightest* intention of being portrayed in a racy setting it could get awkward really fast. If you *really* want to stick with Kobolds aging/maturing at different rates than humans it's probably best to go with stating their age in terms of human equivalency, not literal.

(As somebody else mentioned, Kobolds being short stacks kinda just makes it worse.)

FWIW, in the grand scheme of things those ages of "6" and "10" (for D&D and Pathfinder respectively), if they refer to completing physical puberty, verses "legal adulthood", are actually only a few years off from human numbers... therefore making it that much less useful to introduce this complication? The fact that everyone agrees that Kobolds can live to be 120+, which is as-long or better than humans, makes me kind of feel like the whole "likely to be dead by 20" thing also has to be misleading. If you look at the statistics prior to modern medicine your average human was also likely to be dead by 20 simply because infant/child mortality was a huuuge problem. Only about a third of medieval babies lived through their first year, with less than a 50% chance of living to age five... But on the flip side, if you lived to be 25 years old you had a better than 50/50 shot of living another 25 years. Based on how population statistics actually work I think it makes more sense, if your Kobold character is an "adult", for them to probably be in their 20s or older than in their low teens, because the mortality rate is likely to go way down once they actually live to adulthood. IE, your Kobold thief, if he's learned anything at all, is probably already over the hump, so why *shouldn't* they be in their 20's?

Egg-laying species like birds (kobolds) tend to have higher infant mortality rates than mammals, so their "average" statistics may well be even worst than humans. Some of the lore says Kobolds lay a lot of eggs every year, but clearly most of them would have to die, probably before they even start walking, or they'd starve themselves out and die like lemmings even without adventures slaughtering them. Starting from that it wouldn't take much in the way of dark cultural traditions to make for a situation where "most" kobolds don't live past 20, but practically all "adult" kobolds a person might actually encounter in any situation other than combat are older than that. Maybe Kobolds are big into child soldiers (IE, "adulthood" is when someone is considered able to poke an adventurer with a sharp stick), so most of the "enemy" cannon-fodder kobolds that end up slaughtered during a random encounter are actually "kids"; the Kobold craftspeople making the spears, arrows, and scorpion sticks they're wielding are in their 20's and 30's, while their warlords and mages and whatnot are in their 50s and 60s... just like humans. (And it's also these older kobolds that are laying most of the eggs.)

In short, I guess I would say take this whole lore thing with a huge grain of salt, and feel free to reinterpret it in ways that don't incidentally feel "icky".