This article compares Mandarin Chinese (MC) proficiencies of bi/multilingual children from immigrant families in Canada (BM) with those of monolingual (bidialectal) children (M) from China across two age groups (5–7 and 10–12). The purpose of the comparison is to identify the threshold of mother tongue to heritage language shift among Canadian bi/multilinguals. The results of bivariate ANOVA analysis of 28 speech parameters in children’s narratives elicited with the help of picture prompts demonstrate significant effects of language and age groups on proficiency parameters as well as interactions between language and age group factors. In the younger age group (5–7), bi/multilinguals proficiency is higher in six parameters, and lower in eight parameters, as compared to monolinguals, i.e., language proficiencies of bi/multilinguals are overall on par with those by monolinguals. By contrast, in the older participant group (10–12), there is a higher number of significant differences between BM and M participants (20 out of 28 parameters), and monolinguals have higher proficiencies in all these parameters. Furthermore, within the language group, cross-age comparisons display a significant improvement in 16 speech parameters for the older monolinguals group as compared to the younger group, but none at all for the bi/multilinguals older versus younger groups. The results suggest that for children from immigrant families in the given location and settings, the threshold of Mandarin Chinese shift from a mother tongue to a heritage language occurs between the ages of 8–9 and is largely completed by the age of 10–12, as evidenced by lack of further development of speech parameters in the 10–12 BM group. These results are explained by the impact of majority language education and other social factors.