Service planning for people with younger onset dementia (YOD; an onset of symptoms before the age of 65 years) relies on prevalence estimates, with existing models based upon older people. This pilot study investigated the prevalence and causes of YOD in a defined catchment area of Eastern Sydney, Australia.

The study was conducted in three stages: publicity building, case finding, and case validation. A brief structured questionnaire was sent to health professionals in the catchment area asking how many patients with YOD they had seen over the previous 12 months. Memory clinics and hospital records were also searched for YOD patients. Clinicians assigned a Statistical Linkage Key to each patient to prevent double counting, and indicated the cause of dementia. The majority of patients were validated by a review of medical case notes. Prevalence data were calculated for the following age groups: 30–64, 30–44, and 45–64 years.

Two hundred and four potential patients were identified, of which 141 met inclusion criteria. The primary clinical subtypes were alcohol-related dementia (18.4%), Alzheimer's disease (17.7%), vascular dementia (12.8%), and frontotemporal dementia (11.3%). Eighty-eight patients were aged 30 to 64 years on census date and were therefore included in the prevalence calculations. The overall prevalence was 68.2 per 100,000 population at risk for the 30–64-year age group (95% Confidence Interval (CI): 54.9–83.4); 11.6 per 100,000 for the 30–44-year age group (95% CI: 5.3–21.7); and 132.9 per 100,000 for the 45–64 age group (95% CI: 105.8–164.2).

Younger onset dementia affects a significant number of people in Eastern Sydney with a diverse range of clinical types. This prevalence rate is higher than previous reports from the United Kingdom and Japan, with a different distribution of etiologies, which have important implications for service planning for this group.

The extent to which specific factors influence diagnostic delays in dementia is unclear. Therefore, the aim of the present study was to compare duration from symptom onset to diagnosis for young-onset dementia (YOD) and late-onset dementia (LOD) and to assess the effect of age at onset, type of dementia, gender, living situation, education and family history of dementia on this duration.

Data on 235 YOD and 167 LOD patients collected from caregivers from two prospective cohort studies were used. Multiple linear regression analysis was performed.

The duration between symptom onset and the diagnosis of YOD exceeded that of LOD by an average of 1.6 years (2.8 v. 4.4 years). Young age and being diagnosed with frontotemporal dementia were related to increases in the time to diagnosis. Subjects with vascular dementia experienced shorter time to diagnosis.

There is a need to raise special awareness of YOD to facilitate a timely diagnosis.

We aimed to compare the rate of cognitive decline in patients with early and late onset Alzheimer's disease (AD) and to investigate the potentially modifying influence of the apolipoprotein E (APOE) genotype.

We included 99 patients with early onset AD (age ⩽65 years) and 192 patients with late onset AD (age >65 years) who had at least two scores on the Mini-Mental State Examination (MMSE) (range 2–14) obtained at least 1 year apart. Linear mixed models were performed to investigate the rate of cognitive decline dependent on age at onset (AAO) and APOE genotype.

The mean (s.d.) age for patients with early onset AD was 57.7 (4.5) years, and 74.5 (5.1) years for patients with late onset AD. AAO was not associated with baseline MMSE [β (s.e.)=0.8 (0.5), p=0.14]. However, patients with early onset showed a faster decline on the MMSE [β (s.e.)=2.4 (0.1) points/year] than those with late onset [β (s.e.)=1.7 (0.1) points/year, p=0.00]. After stratification according to APOE genotype, APOE ε4 non-carriers with early onset showed faster cognitive decline than non-carriers with late onset [2.4 (0.3) v. 1.3 (0.3) points/year, p=0.01]. In APOE ε4 carriers, no difference in rate of cognitive decline was found between patients with early and late onset [β (s.e.)=0.2 (0.2), p=0.47].

Patients with early onset AD show more rapid cognitive decline than patients with late onset, suggesting that early onset AD follows a more aggressive course. Furthermore, this effect seems to be most prominent in patients with early onset who do not carry the genetic APOE ε4 risk factor for AD.