P/E's aren't based on year of production, never has been.
Many junior to mid sized companies trade on P/E's much higher than 15.
Using your method in calcualting what should be a reasonable P/E why is BHP trading at a current P/E of around 12?
They will still be producing in 100 years so why not 100?
That is why I say that P/E's aren't based on years of production that a company has left.
It really all boils down to the sentiment of that company.