Among the primary teeth, the mandibular canine typically remains retained the longest before exfoliation. This is primarily due to the eruption sequence and the timing of the permanent successors.
- Primary teeth exfoliation occurs when the permanent successors erupt, resorbing the roots of the primary teeth.
- The mandibular canine's permanent successor erupts relatively late, usually around 9-12 years of age.
- In contrast, the first molars are typically exfoliated earlier because the permanent first molars erupt behind them without resorbing a primary tooth.
- Maxillary lateral incisors and maxillary first molars have earlier eruption timelines for their permanent successors, leading to earlier exfoliation.
- The mandibular second molar exfoliates after the mandibular first molar but still generally exfoliates earlier than the mandibular canine due to its permanent replacement timeline.

Thus, the mandibular canine remains retained the longest among primary teeth, often showing a longer period before it exfoliates.

Reference: Orban's Oral Histology, Volume 5, Chapter: Development and Eruption of Teeth, Page 112-115.