In England, the rate of babies born to women aged 45 and over is up by more than a third in six years, analysis of official statistics has revealed.
The Office for National Statistics (ONS) said there were regional "social, economic and cultural differences". The number of babies born to mothers under 18 is half what it was in 2009, down from 11,135 (11.8 per 1,000 women) to 5,788 (6.3 per 1,000 women) in 2015.
Prof Adam Balen, chairman of the British Fertility Society and spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: "The age at which women are having their first baby has increased over the past few decades due to a variety of social, professional and financial factors, and this trend is unlikely to be reversed.
The data [also] shows that London had the lowest birth rate for women under 18 in 2015, with 4.4 births per 1,000 women. ONS statistician Nick Stripe said London had lowest under-18 birth rate but the highest birth rate for women aged 45 and over.
"In contrast, the north-east of England has the highest under-18 birth rate but the lowest birth rate for women aged 45 and over," he said. "Social, economic and cultural differences between these areas are likely to be causing these differences."
The ONS said the rising number of older mothers was down to advances in fertility treatment as well as more women in higher education and attitudes around the importance of a career. Jacque Gerrard, director for England at the Royal College of Midwives, said: "All women deserve the very best care, regardless of the age at which they give birth. Women have every right to give birth later in life, and we support that. But typically older women will require more care during pregnancy, and that means more midwives are needed."
She said there were benefits to mothers who give birth later, including "having life skills, more confidence in their abilities and a tendency to be financially more stable".