I am going to Kefalonia, Greece for two weeks and was wondering what the age limit is for drinking alcohol and smoking?

In Greece, the Drinking age is between 17-18, while Smoking age is 16, but keep in mind that these Laws in Greece are not strictly enforced
While you do technically have to be the required age to purchase the products, most of the time – especially during the summer when many of the tourists are around – they will not even bother asking for your age. This typically happens in most Bars/Night Clubs. So, if your looking to experiment on these, it should not be a problem but please keep in mind that while most Clubs will let you go, others may not and ask for Identification.
Just keep in mind:
Greece has enacted an EU-regulation friendly smoking ban for public places in Greece beginning September 1st, 2010