At 18 I had been with a guy (i can't even call him DP, cos when i look back he was just a boyfriend!), for a year whom I adored, we were soul mates, i knew what he was thinking, he knew what i was thinking, we were joined at the hip....it lasted 5 years.
At 18, I am sure I didn't know what i wanted, but I thought i did, i thought that was it, marriage, kids, the works. We split up, grew apart, I moved, met DH, married, had twins! I'm now 31 and i adore my family but I'm glad I lived my life the way I did. I'm grateful for my memories, I'm grateful I had my 20's with no kids, because now, I have my boys, my friends of a similar age have their kids, we all still meet up and have fun, get drunk, share memories because we have matured at the same rate. we plan things that involve children, rather than if I had had kids at 18, I feel I would have missed out on sharing fun times with my mates (no not just snogging random blokes in bars - although that does sound appealing sometimes!)....they would be having their kids now and mine would be 12 /13! I'd be wanting to go out for meals and no one would come cos of babysitters etc.
Friends mean a lot to me (my family is miles away so they are my substitute), and when boys were first born it was important to have friends to talk to, friends who knew what having small children was like else you can feel VERY isolated. To be honest i still found the first year or so hard, 1) because i had twins and 2) because my mates were all had their kids slightly later, so were still partying when i was spending 8 weeks in hosp with my two premmie boys.
No one is saying having kids is shit, no one would change their kids for the world, but some of us would change the order they did things in, if given the choice. They see you as having that choice so are using their experience to advise you because you can't get those years back once you have made your mind up.
Even though I had mine at 28, I still didn;t feel ready when they came home - i loved being pregs, was relishing the idea of being a mum, then reality hit like a tonne of bricks, it was the hardest thing I have ever had to do. i went into meltdown and yes I had PND, still on meds now, but I can't change things, I've had them now so you deal with it as best you can - I think what people are doing on here is just hitting you with a hard dose of reality. When I was pregs i had all the 'oh my god the sleepless nights' and i'd laugh and think oh it can't be that bad....it bloody is, but i also know from experience you won't believe us until you try it for yourself.
Just beware having kids changes the whole dynamics of a relationship, especially if its a new relationship (me and dh had been together 2 years when we got married, and boys arrived a year later). You can't be self indulgent with each other anymore and you DP will suddenly be second in the pecking order and he might not be ready for that - your feelings for your baby will be so strong and fundemental you will question your feelings for your DP (sorry, thats just coming from my experience), that on top of feeling overwhelmingly tired, all sorts of things run through your mind. My twins are now three and we are a very strong unit, but we had a very shaky start!
Good luck in all you decide - but please do take on board everything we have said, its not all bollox, it's real life stuff thats happened to real life people.