As an "older" parent (I delivered my daughter when I was 43), I laughed when I read your question. I think it's in people's nature to feel what they're doing is "right" or "best" and you, just as much as your older counter-parts, need to find fault with other people's choices as a way to justify their own.
That's said, I think the BEST time in your life to have a child is when you plan for one and you feel you are emotionally, financially and spiritually prepared to be a good parent. That has nothing to do with "age group" but more to do with the individual.
From the way you describe yourself, it sounds like you are pleased with your parenting and your choices. If I, on the other hand, would have had a child at 18 (God help that child!) I would have been a TERRIBLE parent. I was drinking and experimenting with all kinds of things, I was indulging myself in a wild collegiate experience, I was traveling the world with a backpack, in short, I was more concerned with my own personal experiences and growth and I would have resented a baby for interfering with that. I was selfish and self consumed with my own "stuff" and I didn't have much room in my life for the demands of a child. I don't regret my choices for a moment. I have a rich and lovely catalog of memories from those experiences that helped shape my life in wonderful ways. I went on to establish myself in a fulfilling and socially meaningful career where I excelled in my field and was well rewarded.
So in contrast, I started my family in my 40's. My child is perfectly healthy. I have the luxury because of my choices to be a stay at home mom and I'm outside with my girl daily, hiking, swimming, running, biking, etc. I don't feel like I'm lacking energy and in contrast to many of the younger mom's at the playground I don't feel like I'm "missing out" on a career because of my child, I don't have angst about "what I'm going to be." Like you, I feel very content with the choices I've made and how I'm able to provide for my child and my ability to provide for my child as a loving parent.
Personally I hang out with both older and younger moms. There are good older moms and not so good older moms.. there are good younger moms and not so good younger moms... I'm sure you must know young moms who, frankly, suck at the motherhood thing too. It's not about age, it's about what you're ready for.