I've added a small extra feature that recalculates if a result is older than a certain age.

@memory.cache(expires_after=60)
def f(x):
 ...

Will recompute if cached result is older than 60 seconds. Code is in this branch:
https://github.com/fredludlow/joblib/tree/expires-after

Is this of interest? And if so, is there anything else that needs doing before sending a pull request? (I've added test coverage in test_memory.py)