Q:

Sentences begin with 'that'

just encountered a sentence that begins with the word 'that'.  I haven't seen this approach before.  May I ask how we can decipher its structure?  What special emphasis / feeling does this struture convey?  Below please see that sentence.  Thanks! 
//That the girl consented and/or that the defendant believed the girl was over 13 years’ of age are not defences to this charge.//
Source: https://youth.clic.org.hk/en/topics/Sexual-offences/ 

A:

Using the word "That" at the start of the sentence my represent two things. 
1) Referring to an object mentioned previously (the simpler of the two)
For instance: That was easy! ( Maybe referring to a test earlier)
2) Meaning "The fact that"
For instance: That the food was smelling bad was not in doubt.  
The word "That" here can simply be replaced by "The fact that". 
In your sentence, it can just be interpreted as "The fact that the girl consented and/or the fact that the defendant believed the girl was over 13 years’ of age are not defences to this charge." 
However, this way of using "That" is not frequently used, because strictly speaking, you shouldn't use "That" at the beginning of a sentence.   I would think that different parts of the world would use English differently though.