This article, based on the electronic Corpus of Early English Correspondence (CEEC), argues that adults change their language, at least in quantitative terms. By studying the shift from -th to -s of the third person singular suffix in Early Modern English, the author shows that the relative frequencies of the two variants underwent changes within individual lifespans. The article also claims that the shift followed both the communal and generational patterns of change. Historical letters are shown to be excellent material for the study of language change from a longitudinal perspective, because many such letters have survived for long periods of time. On the whole, letters form good data for research in historical sociolinguistics, since the backgrounds of the people involved (both as writers and as recipients) can be traced and used for analysis.