Final answer:
A T-unit is an analytical tool used to measure syntactic complexity in older children's language, which is crucial in assessing their language development during the L4L stage.
Step-by-step explanation:
A T-unit, or a minimal terminable unit, is an analytical tool used in the study of language development. It can be thought of as a main clause plus any subordinate clauses or non-clausal structures that are attached to or embedded in it. We use T-units when analyzing language samples of older children in the Language for Learning (L4L) stage because they are considered an effective measure of syntactic complexity, which can reflect a child's language acquisition progress. In the L4L stage, as children have usually mastered basic sentence structure, T-units help educators and researchers to gauge more subtle aspects of language competence, such as the ability to construct complex sentences and the capability to express more nuanced ideas and relationships between concepts.