I'm 6 months preg. with DD no.4, and my youngest is 15!
Had them v.young with XH. They all still live at home with me and DH, their Step Dad (eldest, 20, splits the week between here and BF's house actually) and all 3 are really genuinely excited about the new addition :) - much more than i would have been at that age - i was a very un-maternal teen!
This baby will be DHs first child and he is on cloud 9. This pregnancy has bought us both such joy.
I know it's going to feel odd going back into baby mode - and there'll be ups and downs - but frankly i cant wait and neither can he. I've had a nice long break from nappies, pushchairs, car seats and spelling tests and i feel ready to embark on it all one more time. DH hasn't done the baby stage before and knows this will probably be his one biological child so to all the sleepless nights he says ''bring it on!'' 
Every one's situation is different - there's no right or wrong. I think if you know for sure that it's what you and your DP want, and you've thought it through, OP, then it will work out.