we teach in türkiye-anatolia ,Natural Numbers to start at 1 and not to include zero..

but, I could never understand how one could not say that 0 were a natural number

if zero(0) is a Natural Number,why?

someone said:It depends on the frame of reference you use.

right?

It's just how "natural number" is defined. If you want some rationale, it's because natural numbers come from counting and ordering. In ordering, "first" comes first, clearly. Counting is less clear, since you can have zero of something, but you still wouldn't normally say that. In a list of the things you see, you wouldn't start listing the infinite categories of things you don't see. "I have 3 apples, zero unicorns, zero two-headed clowns, zero flying spaghetti monsters...."

As Sisyphus has said, it will depend on the definition you want to use.

 

"Classically" one does not include zero as one "starts to count" from 1 and more fundamentally the notion of zero is much younger than the notion of counting.

Note that there is no zero in Roman numerals.

Whether nor not zero is a natural number is a bit fuzzy. This is stated up-front in the wikipedia (http://en.wikipedia.org/wiki/Natural_number) and mathworld (http://mathworld.wolfram.com/NaturalNumber.html) articles on natural numbers.