The increase in population : Instead of comparing the ratio or the percentage of working vs non-working with the present population it compares x out of every 100.
The increase in jobs : 2000 vs 2014, the number of jobs available has also changed which is not taken in account. As mentioned in the article older people end up creating more jobs.
The increase in unemployment rate : Yes, this contradicts with the above mentioned increase in jobs point but this should also be a factor. This also is affected by people not-from US who work in US.
The Shift in Generation : 14 years brings/builds a very different minded person and this person might have a different approach to living.
The higher standard of living : This also is one of the reasons why older people are still working and won't leave their jobs as their value is appreciated, helps pay the debt and they could satisfy all their needs to live a life of luxury(?) and fulfil all their ambitions.