Average age of first child

em
So I've being doing some reading, and I've found that in every country, the average age a woman is when she has her first child varies quite a bit.
For example, in Angola, the average age is 18. In Greece, its 31.2.
I can understand how places like previously mentioned can vary greatly, but why do you think the following are so different?
- Australia 30.5
- Canada 28.1
- NZ & UK 27.8
- USA 25.4
The numbers don't seem so drastic until you put Australia & USA side by side.
[Info taken from Huffpost. I used the above examples as from what I can see, the majority of users come from those places.]