This analysis aimed to show whether symptoms of either pole change in their persistence as individuals move through two decades, whether such changes differ by age grouping, and whether age of onset plays an independent role in symptom persistence.

Participants in the National Institute of Mental Health (NIMH) Collaborative Depression Study (CDS) who completed at least 20 years of follow-up and who met study criteria for bipolar I or schizo-affective manic disorder, before intake or during follow-up, were divided by age at intake into youngest (18–29 years, n=56), middle (30–44 years, n=68) and oldest (>44 years, n=24) groups.

The persistence of depressive symptoms increased significantly in the two younger groups. Earlier ages of onset were associated with higher depressive morbidity throughout the 20 years of follow-up but did not predict changes in symptom persistence. The proportions of weeks spent in episodes of either pole correlated across follow-up periods in all age groupings, although correlations were stronger for depressive symptoms and for shorter intervals.

Regardless of age at onset, the passage of decades in bipolar illness seems to bring an increase in the predominance of depressive symptoms in individuals in their third, fourth and fifth decades and an earlier age of onset portends a persistently greater depressive symptom burden. The degree to which either depression or manic/hypomanic symptoms persist has significant stability over lengthy periods and seems to reflect traits that manifest early in an individual's illness.

The authors used results from a 20-year, high-intensity follow-up to measure the influence of ageing, and of age at onset, on the long-term persistence of symptoms in major depressive disorder (MDD).

Subjects who completed a 20-year series of semi-annual and then annual assessments with a stable diagnosis of MDD or schizo-affective disorder other than mainly schizophrenic (n=220) were divided according to their ages at intake into youngest (18–29 years), middle (30–44 years) and oldest (>45 years) groups. Depressive morbidity was quantified as the proportion of weeks spent in major depressive or schizo-affective episodes. General linear models then tested for effects of time and time×group interactions on these measures. Regression analyses compared the influence of age of onset and of current age.

Analyses revealed no significant time or group×time effects on the proportions of weeks in major depressive episodes in any of the three age groups. Earlier ages of onset were associated with greater symptom persistence, particularly in the youngest group. The proportions of weeks ill showed intra-individual stability over time that was most evident in the oldest group.

These results indicate that the persistence of depressive symptoms in MDD does not change as individuals move from their third to their fifth decade, from their fourth to their sixth decade, or from their sixth to their eighth decade. An early age of onset, rather than youth per se, is associated with greater morbidity over two decades.