As I thought about when I will have kids I realized I thought about a couple important factors. Eventually I decided why not make a spread sheet. This kind of discussion would probably be social taboo because of the utilitarian nature of the discussion outside of the internet, but I think its valid and probably worth considering in planning your future.

https://docs.google.com/spreadsheet/ccc?key=0AvWc95UG-Q5ldGxIaUNLZzBrZHpjN2FXcVN0c1BtX1E&hl=en_US#gid=0

Explanation.

There are a couple things which are important factors in having a child. Potential birth defects increase dramatically after age 36 for men, and 29 for women. Also, since each person has their own "dating velocity" or how fast they can find a viable partner, and each country has an average age till a marriage, this spreadsheet is dynamic (you'll have to copy and paste into your own spreadsheet program since this is shared and locked).

I put in 2 years. That doesn't mean it takes me 2 years to find someone at all, but 2 years to find someone worthy of a long term commitment. Even if you're tall, tan and jacked, you'll still have to cycle through partners to find someone who is really long term material. So adjust accordingly.

The 2nd variable is time till marriage. It might be conceivable that you would want to rush into marriage based on timing and what not or take your time. Adjust to taste ;)

Conditional formatting. For the sake of simplicity, and just common sense, under 18 for both male and female is suboptimal. You do a lot of growing up from the time of 18-25. So that area is cautioned for males. From 25 up, men are in a good position with income, discretionary spending and their career. For females, who mature much quicker, a range of 22-29 is optimal, since fertility decreases and birth defects increase afterwords. In addition, around 22 should be the time that they will be graduating from college and entering a career. This gives you a chance to see if this person is really going to make it in the world. Since birth defects from a male after 36 go up significantly, that area is red.

The expected age of conception is simply adding the lead time of finding the right person, and settling down with them to get married and have kids. In this case the lead time is 5 years. We assume that very shortly after getting married is when most people are going to have kids. This adjusts the male age to adjust for the timing of the relationship to plan for fertility and birth defects issues after 36.

So depending on your pace and age, give this some consideration to maximize your fertile utility.