On that I'm actually not sure, Bing. Back when I played WoW, Blizzard had a tendency to leave out such... minor details :P
But going by the WoW wiki, which states that Goblins are a 'fast breeding race', and not to mention that they live rather dangerous lifestyles(if someone thinks strapping live rockets to their boots is ever a good idea, you know they might have a limited lifespan). So I'll assume their lifespan is slightly under that of a human.
Not to mention, you could probably chip away another 20-30 years from the expected lifespan of a Goblin, due to their tendency to live dangerous lives, gamble, ignore healthy lifestyles, and not to mention their crazy tinkering and complete disregard of safety protocols.
I'd assume that they mature almost at the same rate as humans, though maybe a little faster, by 2-4 years or so. Can reach 70 years of age if living healthy, but since many of them never make it, I'd put the expected lifespan at around 40-50(and even them I'm feeling slightly bit generous, hah).
I hope that was a sufficient answer, Bing.
Well Wraith, I think I've made roughly 20 NPC characters/w backstories for this RP, atop of my own two main characters so far. I think if you want to join(which is entirely your choice), you'll easily be able to come up with something else.
Like the magical words go: "Use your imagination." ;)