I haven't read all the replies but I'm going to throw my .02 in: The reason that adults are invited to birthday parties is because if you only invite children then you may not get any gifts! That's been my experience post-witless.
When I was growing up in my non-dub household we only had a few b'day parties IN OUR LIFE! No way were you having a party every year - oh NO! So that's the first thing. Second: When I had my last party all my friends showed up without presents for me. According to my mother "I bet they won't be over here next year eating up all my d**n food and didn't even bring my baby a gift .