Writing my books. It literally saved my life. Once I started writing the novels I'd been long planning, I had a reason to get up in the morning. This was during a time when my Dad was suffering from Parkinson's Disease (since I'd been in High School) and I was helping my Mom care for him. Let's just say that things got really depressing, but writing picked up the whole house as well as myself. My parents saw what I was working, and could at last bring my dream to life.

Thinking about the next book. That, and yoga... and biking (while thinking of the next book)... and walking in the Arboretum (while working out problems with the current story).

I remember my third birthday party, so yes.
I guess technically my first story was this little one-page thing I wrote at Summer School when i was about 6. Something about a man walking down a road.

I got it down to a science. First, plan out the book: Outline the book then stat up the needed characters into my database; oh, and this is after I've already built up the details of the world.
Then once I start, first I do a more detailed outline of the coming chapter (a page of notes, really). Then- Day One start at 7:30m in the morning, write about 12,000 words or so, go to health club at about 5, come back and edit until about Midnight. Day two- finish up the chapter, which at that point is usually only around 3000-5000 words so I'm done by Noon; edit what I wrote at night. Day three- what I call my Chapter Edit; only takes about a couple of hours, I give the entire chapter the once-over, then spell-check, followed by outlining the next chapter. 3-day cycle, twice a week. Of course, this is for books the size of Maldene, which typically have around 15,000 words a chapter. Shorter-length ones are 2-day cycles, but either way it's two big chapters a week.
While I write, I play my music. Usually movie soundtracks without lyrics (if you write while playing songs with lyrics then you'll end up paying more attention to the words in the song then the ones in my head)- think John Williams and his ilk. My process has allowed me to average around 125,000 words a month, and the most I've done was writing a 570,000 word book in four months (including editing). enthusiasm helps... a lot.

Technically I started "Maldene" while in my mid thirties... But then there was my first Inspector Flaatphut story in my late 20s... But I really began planning Maldene starting when I was about 19... based on some ideas I had back to when I was 14... but I began observing how other novels were written when I was about 12... though my creative musings date back to when I was about 6... or earlier.
I never have an easy answer to this sort of question. Okay, just say "born to it" and move on.

I'm really great at writing and have some fantastic stories. Unfortunately me and the real world don't always get along; that includes figuring out how to get some legit publisher's attention while not attracting the scam-artists. So, solution iOS do it myself until I can attract a large enough crowd.

The writing itself. Seeing the story come to life, the characters take on their own personalities, and really seeing how it comes out (I'm just as eager as any ready to see the end, even knowing in my notes what's going to happen).