It's not just the next 18 years, it's the rest of your life GA.
Now to your original question, ''when/how did I know I wanted children or not''.
I was on a nice and wild ride with absolutely NO intention of getting married, let alone having kids. Traveling all over the US, Europe, North Africa and the Middle East, I was on a world tour as a night club DJ. That lifestyle comes with absolutely NO commitment other than to the job. I literally would tell the girls I'd meet and get together with that if that phone rang (and it always would) I would pack my bags and go....and no, they could not come with me.
Did that for many years. Life took me to Dubai where I decided to stick around for a bit. Randomly one day walked into an office where I laid eyes on a girl sat behind the reception desk. And I kid you not, it was like I got hit with a thunderbolt. I knew right then and there that that was the girl I was going to marry. Up to that point I had absolutely no intention of EVER getting married. Didn't know her name, nothing about her, and I couldn't tell anyone how I felt, because at that moment I was with my then girlfriend and I didn't feel like getting slapped.
Fast forward a year after our first date, I proposed. We got married about 8 months later. Two years after that, we were sat at a Benihana's having dinner, the question had come up a couple times about kids, and it was at that moment, out of the blue for her, I turned to my wife and said, ''lets do it, let's have kids.''
We were not financially secure or stable. We had NO idea where life would take us. Was it easy, NO. There's a whole load of **** you (both of you) go through, especially once kids get thrown into the equation. Having said that, the night before we got married, I spent about 2 hours in front of the mirror looking myself in the eyes, asking and totally honestly answering questions. I then went to my 'fiance' and had another two hour long chat. I barred all, totally came clean with my past (skeletons in the closet and all), my present and what I thought of the future. Told her to never try to change me, change comes from within when one is ready. A person will never change if being forced to, it comes from within. She still wanted to get married.
Anyway, we're into our 15 year of marriage, two kids (boy & girl 12 & 10).
I am a child of 2 divorces. My slant on marriage is this, 'Marriage is for LIFE!'. Marriage is not a McDonald's drive through. Marriage is not perfect, it takes work, from both sides. It takes give and take, it takes understanding, it takes compromise, it takes open and honest communication, and most important, you both need to LISTEN.
Hobbies come and go....you can always get off and back on that ride. Motorcycles, cars, bikes, RC, fishing, stamp collecting, cooking, flying, etc....they will always be there, whether you are part of them or not.
I've found that the money sorts itself out. I do what I need to do to meet my responsibilities.