as we all know that quatar airways want crew at age of 21 so before 21 can we do job on other airways till 21
Yes, you may associate yourself with Airlines like Indigo, you regularly recruit Crew below 21, followed by Training and Deputation on Routes.
However I would suggest you should pursue your Graduation (B.Sc./BA/B.HM/B.Com/BBA) until 21, after you Pass Out School at 18.
Doing a Certificate Course from Certain Professional Flight Hospitality Training Academy like Frankfinn also increases you chances in getting selected with Airlines.