Our work on age estimation of supernova remnants has been accepted for publication in ApJ. The arXiv link is available. The motivation is that we still cannot be sure about the reliability of supernova remnants’ estimated ages, and thus want to quantify it. We have thoroughly collected available systems in which reliable age estimations were already known or possible. We have compared age estimations which can generally be used for supernova remnants with the reliable estimations for the sample we collected. What we found was that general age estimations were reliable within a factor of four. This uncertainty might seem big, but given the range of known supernova remnants’ ages of a few 10 yr to a few 100 kyr, this constraint is important. And no biases were found such as that certain estimation method always gave larger ages than true ones, which I had suspected to exist.