Abiotic stress is an important source of mortality for cnidarians and is likely to be a major factor shaping their life histories. In freshwater hydra, the ability to withstand exogenous sources of stress varies between species and populations, but little is known about the factors responsible for this variation. Here, we investigated resistance to UV irradiation in Hydra oligactis, a common temperate freshwater cnidarian. We collected polyps from 12 populations and propagated these asexually under standard conditions in the laboratory to obtain 69 laboratory clonal lines with a total of 324 polyps of different age. We measured the size of polyps and recorded their budding rate. In addition, a subset of animals was exposed to hormetic treatment, where experimental animals received a short, sublethal irradiation 2 d before testing their resistance to a higher dose. We investigated how life history traits (age, size, and budding rate), hormetic treatment, and the interaction between life history traits and hormetic treatment relate to the ability of hydra polyps to tolerate high doses of UV irradiation. In multivariate models controlling for the effect of other variables, stress tolerance was positively related to age (lower tolerance in freshly detached buds compared to adult hydra) and size (higher tolerance in polyps with a large body column). Budding rate was negatively associated with stress tolerance. Hormetic treatment increased resistance to UV irradiation, but we found no evidence for an interaction between hormetic response and any of the life history traits, suggesting that the ability to upregulate physiological defense mechanisms after exposure to mild stress does not depend on the life history background of the individuals.