Suppose I have a level 20 Druid age 50, and that I Wild Shape into a beast that lives for, say, 200 years. Now, I'm sure my Timeless Body would apply to my Wild Shape, though if I'm wrong feel free to correct me. The question is, if I was Wild Shaped for 100 years nonstop, would my human body age at all in that time? I'm thinking I wouldn't age, since my body can't age if it doesn't 'exist', per se.
The short answer is, Wild Shape doesn’t say it has any effect on the aging process, so the aging process proceeds normally.
Powers and spells do what they say they do, and you shouldn’t “read in” additional fantastic effects.
Your body does exist while wild-shaped, just its shape changed
The Wild Shape power reads:
If you revert [to your normal form] as a result of dropping to 0 hit points, any excess damage carries over to your normal form.
This only really makes sense if the beast form is actually your transformed body.
Effects that stop the aging process say so explicitly
If a power has significant side effects are specified in the rules, such as Fireball ingiting flammable object.
Timeless Body and Petrification specify that they stop or slow down the aging process. So this is something that the game designers called out, when the effect was intended. (PH p. 67)
Never assume one power includes a more powerful power
Reading Wild Shape to include Timeless Body would make gaining Timeless Body (at level 18!) kind of a let down, if the druid effectively had that power already.
Creatively interpreted lower-level powers should not be short-cuts to higher level powers. For example, the Charm Person spell doesn’t give you the ability to dictate a target’s course of action: that’s what Suggestion does.
So you should take the fact that Timeless Body explicitly slows down the aging process as a cue that the lower-level power, Wild Shape, does not.