There is speculation that some anemones are hundreds of years old based on annual growth rates. However, those aren't necessarily accurate. In reality, it appears host anemones can increase in size quite rapidly in the wild. By the same token, the fact that they don't appear to age suggests that some anemones might be very old indeed. One of the reasons it's so hard to accurately assess how long anemones live is because they don't have any clear markers for age. Animals like mammals age can be assessed because of senescence, but in animals that don't age, there aren't those reference points. It's conceivable that their growth rates slow as they approach a maximum size, but they likely would just very slowly continue to grow or perhaps even stop growing, but not die.