as old as or as young as?

[hoangkha]

Hi, teachers!
I have seen this sentence.
John is sixteen years old, but Mary, his sister, is only eight.

A. He is older than his sister two times.
B. He is twice as old as his sister.
C. His sister is twice as young as him.
D. He is two twice as older as his sister.
The key is B but I am wondering why C is incorrect, please.

[Rover_KE]

It's a matter of usage. We just don't say 'twice as young'.

'He is twice as old as his sister.'

'She is half as old as her brother.'

Rover

[hoangkha]

It's a matter of usage. We just don't say 'twice as young'.

'He is twice as old as his sister.'

'She is half as old as her brother.'

Rover

Thank Rover. But there is a question that
- Is Mars twice as young as earth?

http://wiki.answers.com/Q/Is_Mars_twice_as_young_as_earth

Is it a wrong question?

[bhaisahab]

Thank Rover. But there is a question that
- Is Mars twice as young as earth?

Is Mars twice as young as earth

Is it a wrong question?

Yes, it is.