With my first, I wasn't diagnosed until AFTER she was a year and a half old! It can start anytime in the first year or so.
I'm right there with you - my second is also eight months. It's not the easiest age - I'm back at work, but he doesn't always sleep through the night (he was, but stopped and now it's hit or miss), he's mobile so that's crazy because he never wants to be sitting still anymore and I constantly have to watch him so it's hard to do things like cook if my husband isn't around.

I had bad postpartum anxiety with my son last year and it started when he was around 2-3 months old. If you're feeling really bad my advice is to go talk to someone and see a doctor. I just had my daughter this past April and it's been better but I am starting to feel symptoms again now. I'm already on medication though and I see a therapist. I have lots of experience with this, feel free to PM me/friend me if you need some support or someone to talk to :-)

I went on Zoloft after my first baby. I think around 4 months? I was tired and sad and stressed and just couldn't get out of my funk.

Nothing after second baby.

This time around I'm thinking of an antianxiety... (Zoloft seems to the most accepted for a nursing mom - and since it's an ssri it might help with depression and anxiety). I'm waiting to see how bad it gets (sounds like not the best idea, but I've done this before). We're moving states soon and my family here isn't that supportive, and we wont have much of anyone there. So I'm stressed. I'm hoping once we actually sell the house and start checking thigns off our list I'll feel more in control. Right now I just feel like everything is happening and I have no control. But it's not affecting the level of care I give to my kids (I have less patience, but we still go to play groups and eat and everyone gets changed and I'm not crying or stressed all the time). Sigh. Small kids are hard.