A non-homogeneous birth process is used to describe the statistical properties of bibliometric citation processes. The model is analysed under special assumptions. The life-time distribution, special probabilities and mean value functions are used to characterize differences in the ageing structure of scientific literature, the change of citation impact in time and to analyse predictive aspects of reception processes. The results are applied to selected journals representing one field in science and social science each. The empirical part of the study is based on a 14-year citation history (papers published in 1978 and cited 1978-1991). An intimate connection between impact and ageing cannot be observed. However, the ageing behaviour seems to be clearly influenced by field characteristics and by special document/journal types, such as letters and short communications.