hi dear,

well answering your question - can anyone hav C5-C6 fusion from birth, the answer is "YES".

technically it is called congenital synostosis or block vertebra and for your information the location in order of frequency is
1) cervical spine: C2-3, C5-6
2) lumbar spine: L4-5
3) thoracic spine: any section.

regarding cervical spondylosis, in other words it is called 'wear and tear' of the vertebrae and discs of the neck, hence it is most often seen only in older people.

cheers,

thomas