Typically, trolls in World of Warcraft have roughly the same life span as humans and orcs -- they mature at about the same age, get old at about the same age, and "old age" is roughly the same.
Introduced in Burning Crusade, this race of space-traveling eredar who avoided corruption by Sargeras has a story that spans back thousands upon thousands of years, longer than even the history of the night elves -- and with far less definition. If you want a solid list of historical references and events that shape your character, playing a human, orc or even elf would probably suit you nicely. If you want a gigantic expanse of absolutely nothing that you can proceed to fill at your leisure, you might want to look at a draenei.
On the one hand, deciding an appropriate age for your orc may seem like a simple task -- they have lifespans similar to humans, after all. However, orcs are deceptively complicated, largely due to aging hijinks with the original Horde.
When it comes to age and timelines in roleplay, there's some overlap with certain races. Humans and worgen go hand in hand, and the Forsaken were also at one point human. This means that when you're considering creating one of these three character races, you can pretty much look to the same timeline -- with a few adjustments for where said character grew up, lived, or died, of course.