Asiatic black bears (Ursus thibetanus) and sun bears (Helarctos malayanus) in Southeast Asia leave claw marks on climbed trees that provide a cumulative history of their presence and activities, but this record can be difficult to interpret without knowing the age of the marks. We conducted an experiment to estimate ages of bear claw marks by monitoring 212 fresh claw mark sets (most of which we created to mimic real claw marks) on 122 trees from 17 families in Thailand. We categorized marks as looking fresh (presence of woody grit, sharp edges), recent (absence of woody grit), or old (bark growth in the gouges), and estimated the duration of these age categories using Kaplan-Meier survival analysis. Most marks (81%) remained fresh for at least 2 months, but by 3 months, 75% had transitioned to recent (median 2.6 months). By 10 months, 90% of fresh marks became old (median 7.3 months). Wood hardness had no effect on aging rates. Marks created in the rainy season and those on thin-barked trees aged slightly faster than dry season marks or marks on thick-barked trees, but these differences were slight enough that they could be disregarded in population monitoring programs based on abundance of sign. Simulation models we constructed indicated that the density of fresh (or fresh plus recent) sign would more closely correspond with the number of bears in an area than would the density of all sign or the ratio of new∶old sign, because old sign persists for a long (≥24 months) and variable time, so would tend to be a poor reflection of bear abundance. Fresh claw marks also can be linked to phenology and fruit production of climbed trees, so could provide information on bear feeding habits.