One day, in the not-too-distant future, it will be a rule of thumb that if you haven't had your first gray hair yet, you're too young to become a parent.
There is a different between "having kids" and "becoming a parent." Anyone can have kids. But becoming a parent is a staged metamorphosis, and very few people ever truly complete the transition. And the ironic thing is, those that do are usually too late.
And the REALLY ironic thing is, you don't necessarily have to have kids to start becoming a parent. The first few stages of the transition are eventually heaved upon everyone at some point in their lives, even if they've never had or even wanted children.
It's kind of like compulsory 12-step program. Or 7-step program, as the case may be.
1. Realize that you are not as mature as you could be.
2. Understand that finding maturity will lead to greater happiness.
3. Shed fear, pride, and anger.
4. Identify the priorities in your life.
5. Live according to those priorities.
6. Practice sacrificing short-term gratification for long-term happiness and well-being.
7. Recognize the stages of others, and guide them accordingly.
Many people have kids before step 1. Some make it all the way to step 7. The ideal time is somewhere between steps 3 and 4.