A.J.
Well, as the mother of a 1 year old, you already know you're not shortchanging anyone! Cheat them out of what? Competing for dates and fighting over clothes? :) Age offers LOTS of enriching things to your skills. Youth isn't EVERYTHING. I don't know your health habits, but I'm 40, and don't feel a whole lot different than when I was 30-actually no different. I hardly remember being 20, but I didn't think I felt different at 30 than 20...to be honest, my kids help motivate me to stay very healthy since I am older. I had my 3 at 35, 37, and 39. The third was a big surprise. I LOVE IT!
I'm not gonna lie, since my husband always travels and we have a tight budget and I do EVERYTHING for everyone all the time and take them all on every errand-it's A LOT OF WORK. I do not have a tidy house. But I LOVE it. My 3rd is the biggest character of all, way more challenging than my 1st two, and I don't know what I would ever do without her.
Like you, I had a boy and a girl first, so didn't NEED a third for gender variety, but luckily, I got overly blessed, and not a minute too soon (Though I don't think early 40's to be much different than late 30s really). I'm healthier than lots of the younger moms I see at the park. I'm rarely sure if they're younger or not.
Even kids with young parents could be orphaned. Being 60 when your kid is 20..who cares? They don't need you to do acrobatics with them when they're teens (though I intend to still be doing yoga then and always...)
Technically, your kids only "need" you until they are 18 if they are blessed to have parents that long. The rest is icing on the cake. So you had more time to yourself when you were young than you'll have as a grandparent. Such is life. Many of my friends didn't try for their first until they were 40. and my very close friend had her daughter (2nd child) at 43 after trying for years. She's still a hip cool mom and doesn't feel old.
In the old days before all these hyperactive blood tests, older women had babies all the time. My dad's mom was 42 when he was born in the 40's. He's a lawyer and Harvard graduate, still living well consulting the Pentagon in retirement. She died young, but my moms mom, in her 90's, is alive and well and enjoying great grand kids.
You're going to be 60 in 20 years no matter what. Do you want to spend that 20 years with 2 kids or three? There is nothing wrong with either choice, and your 3rd would be blessed. You can't go wrong! I personally wouldn't wait 4 years to start trying though because you like a certain age spread. You gotta compromise in life. :)
Oh, and discipline is key. I could never do this with 3 terrors. An ounce of firm prevention when needed, which is not easy and takes lots of work, makes it possible. I have friends with one child I could never handle because they do the "child centered" thing not the, "build a child to function well in the world and respect others" thing. And finances-they're gonna be thinner. Again, depends what wealth is to you. Love or stuff. Only some people can afford both :) If you can afford both WHAT ARE YOU WAITING FOR??!
ANYWAY! I personally LOVE being my age, and LOVE having 3.
Note on the population: I heard people on average aren't having ENOUGH kids to sustain the work force (hospitals etc) caring for older generations as they age. Most of our friends have one or no kids. NONE of our European friends have kids. There are facts backing up both sides of the population thing. Don't worry about it.