Latest figures from Statistics New Zealand show the population is getting older, with one in five people predicted to be over 65 by the late 2020s.
The change in the age structure is down to people living longer and having fewer children.
Currently, half the population is over 37. By 2061, projections by Statistics New Zealand indicate this age could rise to 44.
Overall, the population is expected to grow but at a slower rate than it has in the past.
Senior demographer Kim Dunston says this is down to a range of factors, including women choosing careers over children and people wanting smaller sized families.
Despite the slowdown, the number of people living in New Zealand is expected to grow from nearly 4.5 million now to five million by 2025 and six million by 2061.