I'm over 30....by several months. I don't know about settling down....I've never started courting, dating or anything yet. If it's the "ultimate goal" of life then why do most live decades beyond the optimal child bearing age and often decades beyond one's children growing up and moving on? Life has no ultimate goal contrary to what Hollywood suggests and some would like to believe. I think the fear is really based on wondering what'll happen if one does change one's mind or losing the window of opportunity, because like me there has been things one regrets long after it's too late. But that's not living, that's wasting time [word I can't think of right now because I'm very tired right now] over the past and things one can't do anything about.
So let me quote Alan Price:
"Memories are only about the past
The present time will never last
The future lies within your heart"
But 30 is not too late to change ones career, start education (no age is too old IMO) or even "settle down" or give birth. However I would say by that age you'll have a better idea of yourself and what you want and maybe how you'll get it - so I'm pretty sure I'm not going to want kids.
If anything you'll probably get less of "oh you're still young you'll change your mind" and such. Just keep going and enjoy life by making new good memories and not ones envying what you might've missed.