Frequency of occurrence in the input is a main factor determining the ease of acquisition in first language learners. However, little is known about the factors relevant for the acquisition of low-frequency items. We examine the use of aspectual markers in a longitudinal corpus of Chintang (Sino-Tibetan, Nepal) children (ages 2;1-4;5). Only 7.7% of all Chintang verbs are overtly marked for aspect. Chintang has three aspect markers, one of which is substantially more frequent than the others. One of the low-frequency markers is positionally and prosodically more salient, appearing at the word-boundary. Using a Bayesian beta-binomial model, we assess the distribution and flexibility of use of aspectual markers in the input and children's production. Our analysis shows that the most frequent marker was acquired earliest, as predicted. For the low-frequency markers, position, segmentability and uniformity are better predictors of ease of acquisition.