I certainly dont think you are too old, nor do I think you will be the oldest parent at school functions. Many women are delaying childbearing these days.. I have so many friends starting familes after 35. I had #3 at 35, and still am considering #4. And yes, the risk does go up, but truthfully is it still a small risk. I have a good friend pregnant at 44 and her quoted risk was around 3%, meaning a 97% chance of a healthy baby. At 40 it is even better. Yes it is higher than at 30, but still. If your OB feels it is ok, and you feel the risk is acceptable, go for it! If you are worried about the genetic issues you can always request to meet with a genetic counselor as well.
IT really is totally you and your spouse's decision. I find it really interesting reading the posts here on how different we all are in what makes us feel a certain family size is the right one. What I have learned is that everyone is an individual and as with everything has different thoughts and considerations. Because one factor(ie taking more vacations) is higher on the list for some, it may not be as important to others. It doesnt make one or the other person more or less selfish in my book. If you feel complete and happy with your decision on family size, then who is anyone else to question it?
But I do agree that when that longing is there it is tough. I dont know that that longing will ever resolve for me. I WISH so much I felt done like so many here have stated. But I dont yet, and I worry I will carry that regret around forever. I think for some women it will always be hard to close that childbearing chapter of their life... I guess I am one of them!
As for me, I have three and lOVE it! Three is quite common in the community I live. I love what our youngest has added to our lives, he is the sweetest, most enjoyable little guy! Yes it is more work, time, energy, money, but for us, so worth it!!!! We would rather have our three kids every day and have the occasional vacation than not have our little guy and go more. Anyway, I love to watch our older kids with our youngest! Sure they fight at times, but what they get from each other is priceless.
Listen to your heart... yes you have to consider your energy level, health, time, money (again, these arent selfish considerations, just facts of life and important to think through- we are in terms of another or not), but listen to your heart too. And think too, if you decide not to have #3, will you feel ok with it, or will it be a regret? I think some nervousness with any big decision is normal.
Good luck!!! Best decision we made!!!!