Hamilton is New Zealand's youngest city, while the Thames-Coromandel region has the oldest residents in the country.
Statistics New Zealand figures reveal that, overall, the country is becoming older - in the past five years the population's median age has increased by more than a year.
In June, half of the population was older than 37 - the national median age - and of the 67 territorial authority areas, only 13 were below the national marker.
Hamilton's median age was 31.7, while the Thames-Coromandel region's was 49.1 - nearly two years older than any other area.
Hamilton Mayor Julie Hardaker said the city's youthful population was largely because of its three tertiary institutions, meaning that a large percentage of residents are students.
"We're also a city that is attractive to families, and families have a lot of children," she told the Herald.
Ms Hardaker said that most Hamiltonians knew they had a young city and were proud of it.
"It bodes well for our future."
Other territorial authority areas with a relatively young population included the cities of Palmerston North (32.4 years), Porirua and Wellington (both 33.6 years), Auckland (34.3 years) and Dunedin (34.6 years).
Statistics New Zealand said in its report on Subnational Population Estimates, released yesterday, that for many areas a low median age indicated a high proportion of children.
Figures also show that New Zealand's population growth has slowed.
In the year to June there was only a 0.6 per cent jump in the number of people living here - the smallest increase since 2001.
Auckland's numbers are continuing to swell with one in three New Zealanders living there.
Statistics New Zealand said the North Island's main cities were driving the island's population growth.
"Collectively, the four cities of Auckland, Wellington, Hamilton and Palmerston North accounted for 98 per cent of the North Island's population growth in the June, 2012, year," the report said.
Meanwhile, the exodus from Christchurch is decreasing - in the year to June, the city lost 4600 people compared with almost twice that the year before.
Population statistics manager Andrea Blackburn noticed some "interesting differences" across the different age groups.
Over a two-year period, the population in the quake-stricken city aged between 0 and 19 years is estimated to have decreased by 9300 people (9.6 per cent).
And Christchurch city's population aged 35 to 49 decreased by 5700 (7 per cent).
"There was a net outflow of children and their parents from Christchurch after the earthquake, and fewer young adults arrived for study," Mrs Blackburn said.
The data also showed there had been some inflows of workers and that people aged 50 and older were less likely than people of other ages to leave Christchurch.By Amelia Wade Email Amelia