If you paid full fare then the agent should just re-issue the ticket as it is their mistake. Another reason to book direct with the airline rather than through a third party. As a matter of interest, which travel agent?
It is perfectly clear that EK child tickets are from age 2-12 and above 12 pay the full price. However, at 14 he/she is still a child and travelling with an adult so there may well not be a problem at all.