You don't have to be 21, I got my first cabin crew job at 18 as do many others.
I agree with VC, the travel and tourism and 'cabin crew' courses at UK colleges are a total waste of time, far better would be to get out there and obtain some decent customer service experience.
The languages certainly would help during the flights to those countries, some pay extra for additional languages. It however, won't have much influence on obtaining the job, far more important is how solid your examples are for the competency based interview. The recruiters will want to know specific examples of times in the past when you have provided excellent customer service, they will also want to see evidence of you having researched the airline and know the job well.
Good luck!