West Somerset has the highest average age in the UK at 52, with almost three in 10 people there aged over 65, a population survey has revealed.
The figure could reach 56 by 2018 as people continue to retire to the coast.
North Norfolk comes next at almost 51, followed by Rother, Christchurch and East Dorset on the south coast, the Office for National Statistics says.
The ONS has launched a mapping tool to chart UK age patterns, coinciding with this year's Older People's Day.
Last year's government-led event saw more than 50,000 people take part in 900 local activities.
The fastest population increase has been among those aged 85 and over - doubling from 600,000 in 1983 to 1.3 million in 2008, the latest UK figures also showed.
And the population aged 65 and over rose by 1.5 million over the last 25 years - in part driven by greater life expectancy.
OLDER PEOPLE'S DAY 2009
Bupa talent shows in Scotland, England and Wales
Underground guided tour of the Victoria Tunnel in Newcastle
Age Concern tea dance in Blackburn
Bilingual cultural competition on Anglesey
Mass Observation Archive talk in Lewes, East Sussex
Fashion show in Croydon
Older people roadshow in Colchester
A newborn baby boy is now expected to live 77.2 years and a girl 81.5 years, and while women continue to live longer, the gap has closed from six years in 1983 to just 4.3.
ONS senior analyst Chris Smith told the BBC the shift was down to a combination of environmental factors such as cleaner cities and the decline in smoking.
When it came to the youngest average age, university towns topped the list with Oxford and Cambridge followed by Newham in London, Manchester and Nottingham.
Men and women aged between 50 and state pension age were taking a more active part in the work place, the statistics also showed.
The numbers working rose from 66% and 59% respectively in 1992 to 72% and 71% from May to July this year.
This year's Older People's Day involves hundreds of community events across the UK for the over-50s or 65s, including music, dances, talent shows and concerts.
The government-led project, designed to "bring generations together", involves mainly free or low-cost activities.