We investigated the effect of optionality on the acquisition of new/given markers, with a special focus on grammaticalization as a stage of optional use of the emerging form. To this end, we conducted a narrative-elicitation task with 5-year-old children and adults across four typologically-distinct languages with different new/given markers: English, Hindi, Mandinka and Spanish. Our starting assumption was that the Hindi numeral ‘ek’ (one) is developing into an indefinite article, which should delay children’s acquisition because of its optional use to introduce discourse referents. Supporting the Optionality Hypothesis, Experiment 1 revealed that obligatory markers are acquired earlier than optional markers. Experiment 2 focused on Hindi and showed that 10-year-old children’s use of ‘ek’ to introduce discourse characters was higher than 5-year-olds’ and comparable to adults’, replicating this pattern of results in two different cities in Northern India. Lastly, a follow-up study showed that Mandinka-speaking children and adults made use of all available discourse markers when tested on a familiar story, rather than with pictorial prompts, highlighting the importance of using culturally-appropriate methods of narrative elicitation in cross-linguistic research. We conclude by discussing the implications of article grammaticalization for common ground management in a speech community.