Cursive handwriting - how, when, why?

I haven't spoken with elementary school teachers about their method/s of teaching handwriting,
although I think they start with printing, and at some point transition students into cursive, or longhand.  I recently met a fourth-grader who could not read handwriting - saying "we haven't had that yet."

I  would like to know how and when handwriting becomes so individual.  If everyone initially learns alphabet printing, and later cursive lettering, when does their individual handwriting become distinct enough that experts can identify forgery?  When you receive a handwritten letter or postcard, how do you so easily identify the sender without looking for an address or a name?

Why do some people use little hearts or circles to dot their 'i's'?  Why is some handwriting extremely illegible, and sloppy?  Why do some writers scrawl boldly, and others crowd their miniature writing into minuscule spaces?  Why slant one way, or the other - prompting experts to decide they are pessimists, or optimists?

I have seen writing by educated adults which looks remarkably like hen scratches - while others have apparently been schooled in calligraphy.

As for the content of what is handwritten - that will take another diatribe - later.