I was 19 when we TTC. I accept now it was for all the wrong reasons (bad childhood, was desperate to break the legacy iyswim) although also DH is older which meant we didn't want to wait. We got married when DD was 17m and DS arrived 10 months later :o
Do I regret it, absolutely not! But I have 19 year olds in the family now and I think holy crap they are SO YOUNG! 
I found the postnatal period very difficult (due mainly to breastfeeding not working well - I felt a failure) and I was very isolated - having turned 20 by the time DD was born, I wasn't eligible for any young mum groups, but among the rest I stuck out like a sore thumb due to being much younger.
Now expecting DC3 shortly before turning 31, I feel more settled. What also helps is that I have now been diagnosed with autism and that explains many of my difficulties. I am much happier in myself now and don't feel the need to pressure myself into toddler groups etc. My nearly 10yo is super excited and the age gap will work well I think. It was a surprise, this one, but DH and I are very happy. He will have the snip as we agree this needs to be the last (financially, my health is quite shit and also he is getting older). I expect many would assume it's the other way round - that DC1/2 were unplanned as I was so young, and this was the planned one :o
Most of my school/college friends haven't had children yet and here I am on my second go round! Some friends are getting broody though :)