This question addresses a large part of the governments of the worlds difficulty now. By that I mean, even here in the U.S. (but especially, from what I read, Greece) the problem is the taxpayers are supporting nearly three of anyone group of public servants receiving a government pension. Example; here, in the U.S., a teacher can retire at 20 years service at any age they avhieve the service requirement. Many of them are only in their 50's, some in their 40's. Their life expctancy is averaging high 70's. So, the school district (here, it would be a local and not national government) is paying for the teacher on the job, and nearly 2 others who are now retired. (Plus, often their medical services package).
And... that would be fine, I suppose, except they are nearly all unionized. So. in reality, it's the unions trying to protect their political power and own incomes (derived from the percentage of salary from the teachers) that is driving the government's financial problem in a severe reccesionary era.
The unions seem to hold even more power in areas of Europe... as mentioned, that of Greece, since it's been adequately demonstrated the unions and one in particular, backed by billionaire George Soros, that's driving the "Occupy Wall Street" mess here in the U.S. that's going on two months old now...