Hi,
good question. The answer largely depends on the following factors:
1) Consultancy (esp. MBB vs. non MBB)
2) Expert Track vs. Generalist Track
3) Country
Since I am a BCGer, let me answer your question from a stereotypic, German BCG hire:
The average generalist consultant in Germany would join after a Masters degree often incl. 0.5 to 2 years experience abroad and/or internships/civil service, which accumulates to 5 to 8 years after high school to join a MBB consultancy.
In Germany, in my experience, most people would join between 24 and 26 years old, respectively with a Ph.D. between 25 and 29.
In UK, US, Asia, etc. the average age is usally younger esp. driven by shorter high-school length as well as more people joining with a Bachelors rather than a Master.
Hope that provides an initial view,
Jan-Philipp
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