Insectivorous bats in their first year of life generally deposit less fat prior to hibernation than older bats of the same species. In the present study we explored the energy expenditures of first-year (sub-adult) and older than one year (adult) Daubenton's bats (Myotis daubentonii) during torpor and their patterns of roost site selection and fat accumulation in an artificial roost site, removing from the equation the effects of differences in aerial foraging behaviour by feeding them on non-aerial prey (mealworms). Sub-adult bats had oxygen consumption during torpor that averaged 2.75 × greater than adult individuals. In an artificial enclosure in which bats could fly freely and choose whether to roost inside or outside of a hollow brick, sub-adults gained body mass at a significantly lower rate (67.8 mg × day−1) than adults (100.3 mg × day−1), despite being fed non-aerial prey (mealworms). The difference in rates of mass accumulation (32.5 mg per day) far exceeded the theoretical influence of different metabolic rates (7 mg × day−1) in torpor. Despite lower rates of mass gain in this artificial situation, sub-adults ultimately achieved the same mass accumulation as adults because they continued to accumulate fat for a longer period, an option that might be unavailable to them in the wild as feeding conditions deteriorate. The rate of body mass accumulation was positively correlated with the time spent utilising the brick roost site, but utilisation of this site did not differ significantly between age classes. These data support the hypothesis that differences in the accumulation of fat between age classes may reflect in part differences in expenditure as well as differences in food intake, but the contribution of differences in metabolism during torpor are relatively small.