You can start being a scientist anytime, as long as you are curious about Science. I recommend you to read and learn with passion, to ask about the Science concepts that you do not clearly understand, and to do experiments like the one mentioned in the answer above (always with the supervision of a trained person). To be a scientist you have to love Science; after this, try to be a diligent student and some day you will discover that you are a scientist.
I've always been interested in biology. When I was in elementary school, I used to wander around our yard looking for different insects and little animals to learn about. I'd often keep them in cages (with food and water) in my bedroom, studying them for a few days, and then I'd release them. I wanted to be an entomologist when I was that age -- an entomologist is a person who studies insects. I grew up on a small farm so there were lots of places to explore, and lots of lifeforms to investigate. We also had a lot of different pets, from horses, goats, chickens, dogs, cats, birds, to lots more. I also entered the local science fair contest when I was in elementary school and really enjoyed it. I still had a lot of pets in middle school, and still explored some creeks near our house, but didn't collect little critters as much. In middle school I learned more about biology from my classes.
When I started high school, I really enjoyed my science classes. I also looked into the local university that was near my high school to see if I could volunteer in any of the biology laboratories. It can be really hard to find a university laboratory that will let a high school student volunteer in it because the people in the laboratory will need to teach the student a lot and that takes a lot of their time. It helps if you know what they are researching and show that you are excited about their research. Good grades also help. I emailed many different professors and finally one said they'd let me volunteer in their lab. I spent the summer after my freshman year in high school volunteering in a university laboratory that studied organisms in freshwaters (such as lakes and streams). I learned a lot! The next summer I thought I'd try a different laboratory, and was able to find one that would let me volunteer -- this one studied the genetics of little worms (called nematodes). I liked this research more, which affected my decisions about college.
Because of the experiences I had in the laboratories I volunteered in, I decided I still wanted to study biology, but I found biology on the microscopic level more interesting. I applied to colleges that had good molecular biology programs and got accepted to the same college that I had been volunteering in, and so I went there because I already knew I enjoyed the environment. I did a double major in Molecular, Cellular, and Developmental Biology (MCDB) and Humanities. It took 4 and 1/2 years. While I was in college I worked in multiple different university laboratories. This helped me when I applied for graduate school. I was accepted by the graduate school I wanted to attend and did research there for nearly 5 years. At the end of that time, I graduated and had my Ph.D. in MCDB! So, it took a little less than 10 years after graduating high school for me to get my Ph.D.
Since graduating, I have become a professional science writer. This means I get to write about the scientific research that other people do, but I don't do the research myself. I have two science writing jobs right now -- one is being a medical writer for a cancer clinic, and the other is writing educational science material for K-12 students at Science Buddies sciencebuddies. It took me many years of education to get here, but it's been worth it!
Here's a great webpage with information on different science careers: science careers