I am 39 with a 21 month old, 6 year old, 8 year old and 17 year old. And I do know what you are going through.
With my 6 and 8 year old children I fit right in with other mom's at school and activities and still do :) These are my core friends.
I find with the moms of my youngest childs peers I don't fit in quite so well. BUT, I fit in even less when my eldest was young. We live in a fairly affluent community and when I took her to 1st grade the rest of the moms were late 30's early 40's (my moms age) and I didn't fit in (I was early 20's and looked 15) and was never included -- it was awful.
Now with the baby I am experiencing some of the same experiences. This time it is not so much about being excluded ... the other mom's are really friendly, I am just not on the same page. I care less about fashion (mine and the babies), I am pretty confident about my parenting skills (they are pretty worried about what is OK and what isn't), I don't like to compare my babies weight, skills, talents or lack of with other babies, and I have a lot less time than many of the other moms, so I find it hard to find the time to join them for the coffee mornings and outings they plan.
As long as I don't get called "grandma" I'll be happy. Oh please don't call me grandma ... don't call me grandma ...