how old is the united kingdom

In 2019, there were approximately 4.66 million people who were aged between 50 and 54 in the United Kingdom, the most of any age group. This group of people were born between 1965 and 1969 are the youngest members of the baby boomer generation, the large demographic born in the years after the Second World War.
In post-war Britain, there have only been two years where the number of live births was over one million, in 1947 and in 1964. The number of births recorded in the years between these two years consistently high as well, with 1955 having the fewest births in this period at 789 thousand.

Wales has the largest proportion of over people aged over 70 at 17.52%, whilst Northern Ireland the smallest at 13.17%. In England, 15.13% are aged over 70. In Scotland, 15.40% of the population is aged over 70.
The current UK population is 66,435,600 according to the Office for National Statistics.

(2018 estimate, published June 2019 and updated June 2020)
There are 8,769,122 over 70s in the UK, according to the most recent official data (2018 midpoint, published June 2019 and updated June 2020).

References:

http://www.statista.com/statistics/281174/uk-population-by-age/
http://www.agediscrimination.info/current-uk-population
http://www.agediscrimination.info/current-uk-population
http://www.eani.org.uk/parents/types-of-school/school-type/primary-4-11-years