When you have cancer in the kidneys would you be in pain?

Not necessarily. Many kidney cancers today are detected when patients have scans (such as CT scan) for other reasons and there may be no symptoms. However, kidney cancer is not very common in a young person 18 years of age. There are also many other explanations for pain other than kidney cancer. .
Usually . Cancer itself is painless. The side effects and complications cause the pain. So makes detection much more difficult. If you are having issues get a full work up.