I would expect it to be 18 years or above only as you would be signing contracts and therefore must be of the legal minimum age to sign a contract before joining the team, plus you must also be 18 for the purpose of seeking insurance.
If you had gone down the EE apprenticeship route, then most are again 18 to 24, however they do have a minority aged 16 to 18.
A lot of places will let you become an Retail Assistant from the age of 15, however you should be at least 18 to take on the role you are looking at.
You can't have a phone contract with EE until you are 18, so you can't be signing contracts for others whilst you are still under 18.
If not done so already, have a look here as there are more than one way to get into EE.
https://jobs.bt.com/EE
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