>When they can choose it, before they were born or they can wait till
>they are 18, to choose religion?
I don't know exactly (Because, I said that before there isn't sharia in Turkey).

>If they wanna change where exactly can they go let say from Iran ?
I don't know too.

>What is general opinion in muslim world about this Sharia 'law' ?

In Turkey somebodies want to sharia but usually (I assume %90-95) don't want.

Actually sharia mean law in arabic. Sharia laws don't write in Kuran. In sharaid countries there is a religious committee. They decide laws.

If folks don't want to sharia there cannot be sharia. Sample in Iran a shah was there (Riza Pehlevi), and Iran people turn overed him. They bring their country sharia. If they want to sharia it's their problem.