As someone bipolar from a well educated and if not from a wealthy background, certainly not from a poor background either, I'd suggest the sheer cost and pain in the arse of getting diagnosed will have an impact on that "later in life" part.

Most mental illnesses (bipolar included) get worse over time if untreated until you end up hospitalised. Then you'll be diagnosed if you're poor. If you have a bit more in the way of resources, you can get investigations into the symptoms earlier in life and treatments (possibly even "off the books" so you aren't recorded as having a mental illness).