The Irish population, on average, is half a year older compared to 2006, but the number of children aged four and under has risen by 18pc, according to new figures from the Census 2011.
The figures also showed that the number of young adults aged 19-24 has decreased by 12 per cent since 2006, the only age group to show a fall in numbers, while the population of elderly people, aged 65 or over, increased by 14.4 per cent
The report, entitled Profile 2 Older and Younger – An Age Profile of Ireland, and drawing from the 2011 Census, showed that the average age of the population of the State as a whole increased to 36.1 years since 2006.
However, the number of pre-school children (aged 0-4) in the State increased by 17.9 per cent compared to overall population growth of 8.2 per cent. As of April 2011, there were 356,329 children in this age group.
The increase in this age group was even more marked in counties Laois (37.1 per cent), the largest increase for any county, and Cavan, which registered an increase of 30.2 per cent.
In 2011 the population of almost all counties showed an increase in age, topped by Limerick city where the average age increased by 1.5 years, according to the CSO report.
However, a few counties had a younger population than in 2006 with the average age in Laois falling by half a year, and Cavan and Longford both showing slight decreases.
There was an increase of 12pc since 2006 in the number of children in the 5-12 year age group to 504,267.
County Laois experienced the highest growth in this age group with an increase of 28.9pc followed by Fingal (28.3pc). There was a decrease in this age group in Limerick city of 9.4pc and in Cork city (7.9pc).
The population of secondary school children (aged 13-18) remained almost static over the past five years increasing by less than 1 per cent to stand at 344,931 in April 2011.
The highest growth rate was in counties Laois and Kildare with 10.3 per cent, while almost half of all counties experienced a decrease in numbers in this age group.
The population of elderly people, aged 65 or over, increased by 14.4 per cent while the number of persons aged 100 or over was recorded as 389 - an increase of 100 persons on 2006.