It has, I think, become the norm to assume that the fact you have/had shared or gone through the same life stage makes the old-timer an "expert" and the newbie can expect to go through everything in precisely the same way.
Pregnancy has introduced me to a variant to two old pet peeves. First, patronizing, and second, the "I know this because I've been there."
I have received both wanted and unwanted advice from older (not necessarily by age but primarily by experience) mommy friends and relatives, but the most sound advice I've received is from the one closest to my age: Babies are the last thing to go according to plan.
Now, I don't begrudge these older mommy friends and relatives their two cents. Everyone is entitled to their opinion just as I am entitled to choose who to listen to and who to simply nod and smile at.
If there's anything I've decided to really wrap my mind around about this whole thing, it's the fact that it the experience will be different for each woman, even for each pregnancy. It's probably why it annoys me when a non-universal truth--say, a permanent change in shoe size--is insisted upon as fact or if I'm told "Oh, it happened to me so it will happen to you too."
It's always a great deal of fun and quite satisfying to feel that an experience that made you feel alone or even isolated is being experienced or was experienced by another person. But please, let's try not to generalize and, when sharing, emphasize the fact that it was your own unique experience and that your horror stories may not happen to me at all.