Q: Are miles valid on flights I've already taken?
A: Usually not. As a practical matter, the airlines don't want to be burdened with additional work associated with verifying that new members actually took flights prior to enrolling.
However, if you forget to earn miles for a flight you take after you're enrolled, you can get retroactive credit. You may need to submit your flight itinerary, boarding pass, or ticket stubs, so always hold on to these documents until your flight miles have posted.