QUESTION:
We studied that volcanoes are never really extinct, but sleeping.
Is Olympus Mons extinct?  Is there a chance it could be active again?
If so, when?

ANSWER from Jeff Plescia on February 28, 1997:
It is true that no volcano can ever be safely considered extinct.
Although eventually in a region, volcanism ends and it becomes
very unlikely that a volcano would erupt again.

For Mars we have a very poor understanding of how old things are.
We use the number of craters on a surface as an indication of the
relative age (the more craters the older the surface). Because
things are so uncertain we really don't know how old Olympus
Mons is - it may be a few hundred million years old on average.
But parts could be much younger. Because of that uncertainty
and the way geologic processes operate, it could erupt again.
However, we have never seen any indication of an eruption in
any of the images we have ever taken from either spacecraft
or Hubble.