I had my first at 19, my second at 24 and the third at 26 (and I'm at the ripe old age of 27 now :P). One part of me says that I would definitely wait on my first if I had it to do over again, namely because I didn't take the time to truly appreciate the time I had with him as a baby. That said, I wonder if I ever could have realized that until after I had my first, regardless of the age. If I'd waited longer, it may have just given me more excuses to stay in an unhappy relationship, as my son is the factor that finally pushed me to make a better life for me and my family.
All of the growing up I did between his birth and now would have been nice to have first, but I honestly don't know that it could have happened without the experiences I went through to get here. As is, my son pushed me to find a happy, loving relationship with someone who would be a good father and husband. He pushed me to create my own business, allowing me to stay home with him instead of having a daycare raise him while I had to work. Now I'm still young, I'm still energetic, and he's old enough that we can have long, insightful conversations together. At 7, his reading skills are such that we can read books that interest both of us, and then discuss them. I'm really enjoying teaching him how to type and how to write well, and then turning around and playing sports with him. The other two are far enough from him in age that I've learned a lot of valuable things from him that I can now apply to raising his younger siblings, too.
So...yeah, it might have been easier if I'd had him later in life. I might also have been stuck on a specific career path or some track that I'd be too far down to change so easily when his needs became clear. Looking back, despite some drawbacks, I'm glad that things happened when they did. Did I miss out? Maybe, but not on anything that I'll actually miss.