For a professional dance career, dancers generally start at a very young age as their bodies are still growing and are able to adapt to the requirements and aesthetics of ballet. However if you are looking to join in the fun and beauty of ballet as an adult, the limit is boundless.
Age cannot determine your ability.
Ballet is such a complex art form because it is art, at the same time it is also science. Most take decades mastering the technique and skills. Each step or move that you learn throughout the years is a progression to more advanced steps, and there is always something that can be challenged and done better. Ballet education is continuous and you can start anytime.