Don't feel lonely! I'm another teenage Bowie fan, and I've had to suffer persecution (well, light teasing) for it!
Anyway, if I remember correctly, I believe "Five Years" is about a young man coming to grips with the end of the world (we only have five years left to live, hence the song's title).
Take a look at these lyrics if you don't believe me:
"News had just come over, we had five years left to cry in
News guy wept and told us, earth was really dying
Cried so much his face was wet, then i knew he was not lying"
The "narrator" of the song prepares for the end by cramming his head full of everything, even the most mundane details of everyday life (I heard telephones, opera house, favourite melodies / I saw boys, toys electric irons and t.v.'s/My brain hurt like a warehouse, it had no room to spare/I had to cram so many things to store everything in there)
I suppose the reactions of the cop, soldier and so on are meant to illustrate others reactions to the terrible news of the upcoming Apocalypse.