I think he is like 33-35, aroud there. Its about survival for Dexter, he has been able to control the hunger, but when it becomes evident that it has to be done, he always has a few potential victims sort of "on tab". Thats kind of how he explains it. Can't remember if it was in the first or second book. The Ice Truck Killer killed more because he is "born free". He kills without reason or regard. It doesn't matter to him. To Dexter, he has to be absoulutely sure, and even then, in the book, Harry taught him sometimes its best to still not kill, even though the potential target may deserve it. Its about survival.