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The length of the mandibular arch from the permanent left first molar (M1) to the right first molar (M1) typically shows an increase during the mixed dentition phase, which occurs between the ages of 6 and 12. This is primarily due to the eruption of permanent molars and the growth of the mandible. Therefore, the idea that the arch length would decrease during this period is not consistent with the typical patterns of dental development and growth.

During the ages of 6 to 12 years, children experience significant dental changes, including the loss of primary teeth and the eruption of permanent teeth. As the permanent molars erupt, they contribute to an overall increase in the arch length due to the expansion of the dental arches as the jaws grow and change in form. This continuous growth helps accommodate the larger size of the permanent teeth.

In summary, the mandibular arch length usually increases in this age range due to ongoing dental and skeletal development, leading to the conclusion that "increases" would be the accurate reflection of what typically occurs, rather than a decrease or maintenance of length.