To describe changes in the occurrence of periodontal attachment loss (AL) through ages 26, 32 and 38 in a complete birth cohort.

Systematic periodontal examinations conducted at ages 26, 32 and 38 in a longstanding New Zealand cohort study (N = 1037). Periodontitis extent data were used to assign participants to periodontitis trajectories using group‐based trajectory analysis.

Eight hundred and thirty‐one individuals were periodontally examined at all three ages; the prevalence and extent of AL increased as the cohort aged. Between 26 and 32, one in nine participants had 1+ sites showing new or progressing AL; that proportion almost doubled between ages 32 and 38. Four periodontitis trajectory groups were identified, comprising 55.2%, 31.5%, 10.7% and 2.5% of the cohort; these were termed the “Very low”, “Low”, “Moderately increasing” and “Markedly increasing” trajectory groups respectively. Those who had smoked tobacco at all ages from 15 through 38 were at higher risk of being in the “Moderately increasing” or “Markedly increasing” trajectory groups. There was a similar risk gradient for those who were in the highest 20% of cannabis usage.

Periodontitis commences relatively early in adulthood, and its progression accelerates with age, particularly among smokers.