Non-classics (and some simple-virilizers) usually don’t get treated from birth simply because no one realizes that they have this disorder.  When you are a salt-waster, the disorder is recognized early because you can go into adrenal crisis as an infant.  And, if you are female, you may be born with virilized genitalia. 

Failure to thrive and atypical genitalia are warning signs that something is wrong.  Non-classics lack those early tell-tale signs.  They get treated later, simply because they are usually diagnosed later.