Complex sentence: The play is about the threat of "someone else" and the threat of growing old--and how, by spending too long fearing and guarding against these things, paranoia can become a genuine and terrifying reality.
Split into simpler sentences: The play is about the threat of an "Other" and the threat of growing old. It suggests that through spending too long fearing and guarding against these things, paranoia can become a genuine and terrifying reality.