The simple answer to this is they never existed.
The reduction seen across much of England and Wales in the population of the 30 to 44 age range between 2001 and 2011 is a consequence of the low birth rates seen in the 1970’s. The majority of those born in that decade now fall within the 30 to 44 age range and because less people were born in the 1970’s compared to the 1960’s there was always going to be a reduction shown for this particular age range in the 2011 data when compared with the 2001 data. In 2001 the 30 to 44 age range accounted for 22.6% of the total population of England and Wales, by 2011 this figure had dropped to 20.5%. In contrast to this those who were previously aged between 35 and 44 at the 2001 Census have now been combined with the population of the post-World War II baby boom to create the only age range, 45 to 64, that has not had any population decreases over the past decade in any local authority. The 45 to 64 age range accounted for 25.4% of the total population of England and Wales in 2011, a rise of 1.6% when compared to 2001.
Another interesting pattern seen is the population decrease in the 0 to 14 age range. This once again can be accounted for by low birth rates around the start of the millennium. These low birth rates can be seen clearly in the population structure of England and Wales, and to illustrate this point those who were aged 9 at the time of the 2011 Census accounted for 604,000 of the total population of England and Wales – the lowest contribution of any individual age band to the total population count from the ages of 0 to 64. It is not until you reach the 65 age band where a lower contribution – 557,600 – towards the total population of England and Wales can be seen.
Below are maps showing the percentage population change in England and Wales between 2001 and 2011 for Males and Females aged 0 to 14, 15 to 29, 30 and 44, 45 to 64 and 65 and over respectively.