As residents will be aware the government conducted a census of the population in 2021. The results are slowly being released.
The most recent release included detailed demographic information at the level of council wards. The two Selsdon wards are Selsdon and Addington Village, and Selsdon Vale and Forestdale. Their age demographics are included in the attached chart. For comparison the demographics of nearby Sanderstead and the more distant Broad Green are included.
Broadly the wards in the South of the Borough are home to older age groups with wards in the North, like Broad Green, being home to a younger demographic. Most wards see a drop in people in the 19-25 age group as children mature into adults and tend to move away to university or to employment opportunities outside London.
There is much more information in what has already been released, which you can find on the Census 2021 web site.