The Office for National Statistics team says although 65 has traditionally been seen as the start of old age, 70 could be seen as the "new 65".
That's because many people who reach this milestone birthday can still expect to live another 15 years.
Remaining life expectancy may be a better marker of old age, they say.
Traditionally, 65 has been taken as the entry point into old age.
For decades it has been the official retirement age for men when they can start drawing their state pension.
But working patterns are shifting and the pension age is rising for both men and women - it will reach 66 in 2020 and 67 by 2028.