18, but it will vary according to local laws in the territory you are in.
If you mean amateur sumo, there is no minimum. You can start doing sumo for fun anywhere and any time you please. If you mean professional Sumo which exists only in Japan, the minimum is 16, and the oldest you can be when starting out is 23.
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You must have completed 9 years of the education system to be eligible to join a sumo stable. This means an age of about 15 or 16. They then take the six month Sumo Training School, which gets them into shape and teaches them the history and basics of sumo.