This is an interesting discussion, and one that I've chatted about with people in the past. There's a couple of points I think might be interesting to consider.
The age chart is poorly defined. Humans are not adults at 15. That's utter crap. Sure we reach sexual maturity somewhere in that vicinity (I personally grew 6 inches and put on ~60lbs in my 16th year), but sexual maturity and emotional maturity are not equivalent. The prefrontal cortex, the part of the human brain associated with what we would normally call "maturity," doesn't complete growth until around age 27. So there's a basic problem of what do we mean by maturity/adulthood? Are we saying that elves grow more slowly than humans, or that they reach sexual maturity later, or that they reach emotional maturity later? (Here I'm using elves to represent the long lived species). Do these species have the same stages of growth as humans do? Chimps don't. They reach brain and sexual maturity at roughly age 13 (which looks a lot like the orc pattern. Actually, a lot of orc stuff is very reminiscent of chimp society). Chimps don't have that "teen-ager" phase that humans do, where we are more or less physically mature, but not fully formed emotionally. I've seen this used to explain elves before. Elves have a much longer "teen-age" phase, where they are simply too flighty to acquire many skills (although they do acquire some skills, such as the weapon proficiencies). It might also be the case that, while fully grown, they aren't reproductively mature yet, that their fertility doesn't mature until after 100 or so. That would be interesting to explore, but without fertility data...
But what about dwarves? They live a long time and typically aren't described as flighty? Perhaps they simply take 40 years to acquire those racial bonuses? Perhaps it's not til 40 that they can tell the males from the females? It's the genie-kin/outsider-kin that really make no sense. For some of them, they look completely human (or almost) and yet it takes 60 years to reach adulthood?! I think that'd point to something being off pretty damn quick if we were referring to physical maturation.
I think there's another possibility here as well, and it may have been the original point of the chart. Adulthood is culturally defined. These basic ages are meant to be "the age at which members of community X allow their young to go off alone." From that perspective, elves (and other long lived races) do reach physical maturity on a commensurate time line, but take a long time before they're considered full adults. Elves simply see everyone as children before they've got a good century under their belt and DON'T LET their "young" go off on their own. In that case, elves or ifrits or whatever that are raised among communities of different races (like humans) WOULD be considered adults at the same age as everyone else. If that were the case, then an elf raised among humans would be considered an adult at 15 (or so) and off on their own. Since they were raised among a different community, they would loose those racial benefits/bonuses that we see them normally acquiring (things that their normal community would consider essential before letting anyone off on their own).
Of course, I've also had discussions with people arguing that the cause of the extended life spans has to do with magic affecting the rate at which telomeres decay... at which point I like to mention that these are fictional species.
