Special relativity tells us that twins age differently if after birth they part, follow different paths through spacetime and then reunite. The twin who follows the longest worldline ages least. At least this is how I imagine the twin paradox, which I thought I understood. Here's a silly question. Why doesn't a difference in say, height, develop between the twins as they travel? What aspect of the time dimension makes it so different from the space dimensions, even in relativity where dimensions are treated democratically? Is it just the mysterious fact that "time flies" while space dimensions don't?