I am 30 year old and my family fixed marriage with 18 year girl. Is it really odd ?

Where? I understand in Countries where arranged marriages exist this is NOT odd. In fact, sometimes the "brides" are so young that the practice is illegal.

Why do you ask?

A 12 year old age difference is not strange, esp in areas where there are arranged marriages.

Many countries in the world have arranged marriages.

It is considered that the parents know what is best for their child,however in other countries even though there is still arranged marriages, it doesn't mean that the couple concerned do not have their say.

The age range is sometimes quite vast,although in this case I would say very acceptable and not at all odd,taking into consideration the culture.