Interesting post! My two cents ...
I'm 22 and due on Feb 2nd, so guess I can relate. I know people on here are keen on marriage, but seriously, if you're not fussed then don't bother. Getting married for legal reasons is NOT a reason to get married if it's not for you at this point.
In all honesty, pros and cons.
Pros:
I feel I've had it 'easier' than older pregnant women I've met. It hasn't been brilliant, but my body has handled it well.
Both DP and I are glad that we'll be young parents - we're both full of energy and are very active. I appreciate older parents can be energetic too!
I thought I'd lose all my friends but actually, I don't think I've lost a single one because of this. If anything, they are really excited and positive about it all - it's brought me closer to a few of them. That may change however, when baby is actually born and throwing up everwhere!
I do feel more settled and happy with 'myself'. Being pregnant has made me more peaceful, responsible and generally nicer (!) than anything I've experienced before.
Cons:
We'd rather have bought a house first, and in London, even with DP's savings, a baby has now made that impossible in the immediate future.
We'd rather have more money! We both have full-time jobs and very good degrees, and earn approx 60k pre-tax between us, but again, in London with childcare etc, that doesn't go far.
Having a baby young really does tie you down. I think both of us are now thinking, "Wow, this is it. A little person FOREVER."
I do find it hard to bond with other mothers to be, at antenatal classes etc. They are all older, and I feel there may be preconceptions about 'young mothers' floating around.
Can't drink, too tired to stay out late, miss letting loose and wearing tiny clothes and generally being young. The one thing I will say is that I DO feel my 'youth' has been cut short. I don't resent that, and am sure I'll have some fun after baby is born, but it will never be the same. It's a trade-off that is definitely worth making, but do consider it.
Career: depends how important it is to you. It's very important to me: do finish your degree if you can because it all suddenly gets difficult when you have a baby! I've realised that the only way to keep my career on track is to go back to work asap and sacrifice baby time. But that's another trade-off.
So ... yes, pros and cons. More cons than pros! But of course, the big pro is that you get your baby, which is brilliant. I don't think 20, 21 or 22 is 'still a baby' and if it's what you and your DP want, do it. But you're not going to suddenly become unable to have a baby in the next couple of years, so maybe finish the degree first.