English has a class of “semiweak” verbs, which in the past tense have root-vowel ablaut as well as reflexes of the apical stop suffix, for example, kept, told. This study traces the development of this class in a sample of speakers aged 4–65. The evidence is derived from the variable rates of occurrence of the final -t,d in these words in the speech of individuals of different ages. The rate of -t,d absence in semiweak verbs systematically declines with increasing age. We identify three ontogenetic stages in the development of the class. In children's speech, these segments rarely appear, suggesting they are underlyingly absent. In young adults they appear but undergo the variable -t,d deletion process of English at the same high rate as noninflectional -t,d in words like west, old, implying that such speakers do not treat them as affixes. Finally, some adult speakers show a lowered deletion rate, suggesting that they accord the final stops separate morphemic status. The age distribution of this pattern implies that speakers only arrive at this analysis in adult life, after the age when acquisition is often assumed to be complete.