The fertility rate for women aged 40 and over has risen above that for the under-20s for the first time since 1947.
A new report from the Office for National Statistics (ONS) for England and Wales found there were 15.2 live births per 1,000 women aged 40 and over in 2015 compared to 14.5 for those aged under 20.
In 1981, the rate was 4.9 for women aged 40 and over compared to 28.1 for women under 20.
This means the fertility rate among older women has more than trebled since 1981.
The report also said more than a quarter (27.5%) of births in 2015 were to mothers born outside the UK - the highest level on record.
This percentage has increased every year since 1990, when it was 11.6%.
The long-term rise in babies being born outside marriage or civil partnership has also continued, the ONS said, with 47.7% of all babies in 2015 born outside marriage or a civil partnership, up slightly on the previous year.
On the trend of older mothers, the report said: "In most developed countries, women have been increasingly delaying childbearing to later in life, which has resulted in rising fertility rates among older women.
"This may be due to a number of factors such as increased female participation in higher education and the labour force, the increasing importance of a career, the rising costs of childbearing, labour market uncertainty and housing factors.
"Rising fertility rates at older ages have affected the average age of mothers, which has been increasing since 1975, reaching 30.3 years in 2015."
In 2015, fertility rates decreased for women in all age groups under 25, and increased for all age groups 30 and over compared with 2014.
Women aged 30 to 34 have had the highest fertility of any age group since 2004, when it was 25 to 29.
Overall, there were 697,852 live births in England and Wales in 2015, an increase of 0.4% from 2014.
In 2015, women had an average of 1.82 children each, down from 1.83 in 2014.
Elizabeth McLaren, from the ONS, said: "The trend for women to have babies at older ages continued in 2015. Over the last 40 years, the percentage of live births to women aged 35 and over has increased considerably.
"Women aged 40 and over now have a higher fertility rate than women aged under 20 - this was last recorded in the 1940s."
A spokeswoman for the British Pregnancy Advisory Service (BPAS) said: "The trend towards older motherhood is here to stay, and there are many understandable reasons why women today are waiting longer to start or expand their families than those in previous decades.
"Rather than bemoaning this development, we should seek to understand and support the decisions women make. More affordable childcare and improved maternity rights may make it easier for some women to start their families earlier if they wish, but we also need to ensure we have high quality reproductive healthcare services configured to meet women's needs, whatever the age at which they conceive."