There is no limit for radioactive dating. It can be done on a specimen that died tomorrow to specimens aging billions of years. The extent of this age is dependent on the half life of the isotope you are measuring. For instance, carbon 14 has a half life of 5743 years.
Hence it can estimate the age of specimens dating to 75000 years. This is why, we use other isotopes with longer half lives for specimens that date back even further back. One commonly used isotope is the potassium 40 isotope with a half life of 1.25 billion years dating back billions of years.