In 2002, 40% of the population was over 44 years old. Fourteen years later it was 43.5%. The share almost equaled to the population below 35, which was 43.7%. Life expectancy also increased. For people aged 65 years or older, the expectancy increased by more than two years. Their share increased by 2.1% between 2002 and 2016. There were 7.1k centenarians in 2002 and 13.7k in 2016.
I was curious about the white-hot sliver of youth in the wilds of Wiltshire. There are military bases around those parts, but couldn't immediately confirm that's the reason why.