The main economic problem is scarcity - fulfilling people’s want and needs with limited resources. Peak oil theory states that, at one point, this resource will reach the maximum level of production effecting the continuous decline afterwards.
M. King Hubbert, who created and first used the models behind peak oil, initially predicted in 1974 that peak oil would occur in 1995 "if current trends continue."
However, in the late 1970s and early 1980s, global oil consumption actually dropped (due to energy-efficient cars, electricity and natural gas for heating). This clearly shows that the only way to identify the timing of peak oil will be in retrospect.