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 If you want a solid list of historical references and events that shape your character, playing a human, orc or even elf would probably suit you nicely.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.
Warlords of Draenor brought to the table a lot of new story that all roleplayers can use and play with, but it also brought a pretty unique opportunity for both draenei and orc roleplayers. Because this expansion is taking place in an alternate universe version of Draenor, orc and draenei roleplayers now have the chance to play in a world of "what-if." What if your character is not from Azeroth, but in fact originated on this version of Draenor? What if you have two versions of the character you've been playing all along -- the Azeroth version, and the Draenor version? What if the two of them meet?