How old is too old?

I am an active member of at least two "mommy boards".  Sometimes more if the mood strikes me, but I have two that I consider my home boards.  We talk about general mundane topics, politics, ask parenting questions, post news items, and just generally interact like a big group of friends would.  For me, it's a good outlet and sounding board and has also led to some real life friendships that I value greatly.

One of today's posts has me thinking a good bit, though.  We have a large range of ages on the board on which this question was posed.  Moms as young as 23 and as "old" as 43.  I say "old" because I know 43 isn't old at all.  Isn't it funny how the closer we get to a certain age, the younger it seems?  I mean, I'm 31 and I still don't feel like an adult and I have two children and a mortgage.  But I digress.

The question that has me working my noggin muscles was, to paraphrase, how old is too old to have a baby?  Via traditional means, donor eggs, donor sperm, medicated cycles, IUI, IVF, adoption, whatever means gets you a baby.  {Adoption wasn't mentioned in the original question, but I'm throwing it in there because to me, being pregnant and giving birth is only one small, and optional, step in the journey of parenthood.}  Logistically, what is the cutoff for childbearing years?

The older I get, the more my feelings change on this and the more they stay the same.  Right now I am of the mind that we will likely shut down the studio when I turn 35.  We bat around the idea of having a third child, and are leaning more toward it than against it.  I am deliriously happy with my two boys and if they are the only children we have then I consider myself to be blessed beyond measure.  But I love being pregnant and I love having a baby, so I really can see us doing this one more time.  However, being nearly 32, I do feel a bit of pressure being so close to my self-imposed deadline.

So then I wonder why I've put this deadline on myself, when I know women who have had and are having babies well into their forties.  That part of me tells me that at 31, I have all the time in the world.  I do pregnancy very well, and apparently can get pregnant without issue {and without planning} after the year it took us to get pregnant with our first son.  I'm relatively healthy, even for an overweight person.  If all holds steady, I could see being pregnant at 37, 38, 39 years old.

But the other part of me is thinking about sibling spacing, kids in college, and more time to enjoy an empty nest.  My personal preference is to not have a very large gap between siblings.  Our boys are 2 years apart, and I'd like to keep the spacing between little brother and the next child not much larger than that.  With that in mind, I could see us trying for a third child in the early part of 2014.  The baby will turn 3 that September and we would have a baby right around my 34th birthday.  That would put me at age 52 when the third child turns 18, which to me is still pretty young.  And maybe I'm crazy to be thinking about that kind of stuff right now, but it's what pops into my head.  So that kind of reinforces my self-imposed deadline of age 35.

It's not a question with a definite answer, and there are so many different factors that can come into play.  So I guess the answer is that it depends on the individual and their situation.  What do you think?  For you personally, how old is too old to have a baby?  When is your cutoff?