Third in the series: what do you remember as being the most important thing to you when you were fifteen years old? ~

I grew up that year. Mum went into hospital for major surgery, Dad got injured at work and was also hospitalised, leaving me responsible for my 14- and 12-year-old brothers. The younger siblings were farmed out to other relatives, the eldest three were to stay at school so looking after the next two was my job. Not easy, they were both bigger than I was.
My best friend died that year in a railway accident, and I had my first proper girlfriend - and broke up with her. 1984 sucked. 

A much too old for me guy and getting my driver's license. 

Probably getting a smartphone and driving. I was eager for both. I was a typical teen in that regard, but I mostly spent those years studying and reading and being quiet. 

When I was 15?
I remember having a job on the weekends and I remember passing out on a bathroom floor at a party.
I remember playing sports.
To answer your question, I don't think anything was really that important.
That's what was good about being 15. : )

I guess being 15 and one day older.