Congratulations :)
I had my first at 23, and now DH and I are on our 4th (and last!) - I'm 11 weeks now.
Being a younger Mum is fab. I'll admit its sometimes been hard when all the Mums of kids the same age are older (and richer,and in a different 'life stage'), and I have found some (not most, just a few) older Mums have been downright rude to me and seem threatened by the fact that I am younger. The area where I live has mostly 'older' Mums - most having their first over 35.
The other thing I have found hard is the inevitable drifting from some (again, not all) childless friends, just because our lifestyles are so different. But good friends have hung in there and been happy to visit us and have take-away rather than go out, and they love mu kids are a great support.
Its been wonderful to have the energy to be busy with my kids, great to still feel a little bit connected to the 'youth' culture that my kids are growing up in, and great to be able to have the time biologically to have 4 kids with the timing and spacing we wanted (I'm having my 4th at 30 - so still very 'young' around here!).
Anyway, you have lots to look forward to, and the baby will be a blessing to your family and freinds as well as you and your DP. Best wishes!