It's becoming more and more common for couples to wait until they're a little older to have children. There are many reasons for this, including men and women wanting the freedom to experiment in their 20s (and not wanting to feel tied down to a child); not feeling psychologically ready to be parents; wanting to establish themselves and their careers before bringing children into the world; having trouble finding a spouse; and having trouble conceiving.
Waiting to have children until you're older can be a good thing. If you're 35 when you have your first child, you've been an adult for longer than someone who is 25. You have more life experience and more inner resources to draw on in stressful times.
You'll also have your own sense of identity based on your life experiences. Parents who have children when they're young sometimes define themselves based on their children: "I'm a mom" or "I'm Jackie's mom." Older parents are less likely to define themselves based on their children; instead, their children are an additional "feature" to the sense of self they've already developed.
At early-middle age you'll be at the height of your earning power, meaning you'll be more financially stable and better able to support your children and provide everything they need as they grow up. Having children in mid-life will provide you with a sense of renewal, refreshing and energizing you at a time that you might otherwise start to feel down about growing older.
In mid-life, people tend to value life in general more than younger people do. As a middle-aged parent, you're more likely to really value and appreciate every moment you have to spend with your children.