Or are you too old? But is there any actual limitations for learning critical thinking? For example, can a primary school student learn it? Can elderly people start using it?
There are a number of skills inside critical thinking. Some of them require background knowledge. So to use these skills you need to got this knowledge somewhere. It can be a school, or it can be a specialized course. For example, inductive reasoning needs some probability theory, so a critical thinking syllabus can be build assuming students know it already, or elements of probability theory can be integrated into the syllabus.
But probability theory require some more background knowledge, so even if you have a critical thinking course like this you probably can’t teach it to kindergarten kids. In the same time you can replace abstract concepts of probabilities with examples from the real world and use them to explain probabilities to kindergarten kids. Of course creating a critical thinking syllabus for this audience will be more difficult than for adults with degrees but I believe it can be done.
Similarly we can create a syllabus for other social groups with different backgrounds. But the question like “Am I too young” actually may have a different meaning, “Do I really need this stuff?” Imagine a college student who is immersed in a social life: he goes to parties, to clubs, to other non-college events. He may think something like this: “I am too young for this boring stuff, I want to have fun while I can.” So the traditional approach wouldn’t work in this case, you can’t present critical thinking as just another course.
If you want to involve people like this in critical thinking you probably need to rethink the whole approach for education. Some of them may need computer or smartphone games with an educational component. Others may need a social event like a convention or even like a party. So you can find what kind of daily activity your audience wants and create a course or a teaching event that can mimic this activity.
Do you still think that you are too young or too old for this stuff?