This study considers marking of text segmentation in written and spoken monologic texts in the narrative and expository genres from a developmental perspective. Our sample consists of 320 texts produced by schoolchildren aged 9-10, 12-13, and 16-17, and adult university graduates. Our aim was to identify structural positions in a text (e.g. text opening), in order to ascertain their discourse functions in that context (e.g. introduction) and to examine the kinds of discourse marker that occur in that site. To this end, we analyzed marking of the openings of text units called clause packages, which typically consist of a number of clauses linked syntactically, thematically, and/or discursively. Markers were divided into genre indicators, which indicate text type, and content indicators, which also indicate position and function of segments in the text. We found that children and adults both produce different discourse segments in their texts and use different textual markers to signal distinct textual relations. Marking of text segmentation thus reflects how speaker-writers highlight textual content and serves as a good diagnostic tool for describing text quality.