Can someone explain how this situation works in special relativity?

OK, so let's say there's twin brothers, Al and Bert. Let's say Al goes off in a rocket at .9c away from Bert for two years of Bert's time. When Al comes back, he'll be younger than Bert is. I'm fine with that. But now let's say that Al stays put and Bert shoots off in the opposite direction at .9c for two years of his time. Wouldn't Bert, when he comes back from his trip, be younger than Al? But aren't the two situations exactly the same (since velocity is relative)? Obviously, they can't both be younger than the other brother.