The twin paradox in special relativity has been discussed over and over again. Send a twin on a spaceship out to someplace or another accelerating at Earth gravity, then have it go through a series of decelerations and accelerations -- all at 1g -- so it returns to Earth, where the other twin is waiting. And, depending on how long the trip was, the Earth-bound twin is, say, seventy years old while the spaceship twin has aged just a few years.
Within special relativity, it all makes sense. I have done the calculation etc. But what about general relativity? The fundamental observation of GR is, as I understand it, that all accelerating frames with a given acceleration are equivalent. (And I'm betting that my understanding of exactly what "equivalent" means is the answer to my question. But, proceeding ... ) So, the twin on the Earth experiences an acceleration of 1g, as does the twin on the spaceship. Why are their respective frames not equivalent, and they age differently? In a related question, what if we had triplets instead of twins and the third triplet spent the whole time weightless (ignoring health effects) in a space station orbiting the Earth? How would he age compared to his siblings?