So, I found some contradicting information to answer this on the internet and almost none of it was dated to a specific time of the victorian era, so I would like to know if anyone can answer this question properly (also there are further questions in the last paragraph).
- They began wearing them at 6-8 months old. (probably 1860s; Source: https://www.nytimes.com/1997/08/31/nyregion/victoriana-complete-with-corsets.html)
- Well bred children wore them from 3 months old onward. (not dated; Source: https://twonerdyhistorygirls.blogspot.com/2010/01/teeny-tiny-corsets.html)
- They had to wear them at the age of 16. (not dated; Source: http://www.michaelyoungkin.com/childrens-clothing/)
- There is evidence, that children in the 1890s wore corsets from at least one year up as a catalogue advertisment suggests. (1893; Source: https://www.ilsegregoorcostumes.com/wp-content/uploads/2018/10/childrens-costume-research-2-e1539439656913.jpg)
- They started wearing them at the age of nine/ten to acchieve the perfect female body when grown up. (not dated; Source: https://journals.euser.org/files/articles/ejls_sep_dec_17/Melis.pdf)
Further thoughts and questions of mine (feel free to correct me): People (also children) wore corsets both for health/support and the look. But in the late victorian era clothing became less "shaped" (especially when it comes to young girls) and thus the latter reason could probably be omitted. Now, I'm also wondering, if every family (also the working class?) considered corsets for children an essential. Or did some girls (as source 3 and 5 suggest) only start wearing corsets when they hit puberty. Also, at what age did boys stop wearing corsets? When they stopped wearing dresses or when they could walk properly, maybe? Although some men wore corsets, so they didn't stop wearing them necessarily.