Only 23.9% of Greece's population is under the age of 24. By comparison, 47% of Saudi Arabia's population falls in that age bracket.

Greece has an aging population: Only 9.8% of Greeks are ages 15 to 24, while 32.9% are over the age of 55. The birth rate has fallen to 1.3 babies per woman.
This is a twofold problem as rising pension costs will have to be paid for by a labor force that's shrinking.
Source: CIA Factbook