Hear hear cabinboy!
True, you can get in at 18, and a majority of airlines do hire people at this age, but the trend at the moment is to hire people older than this (those with experience in the customer service or hospitality fields).
Some airlines have learnt, as have new hire f/a's, that it takes more than reading and writing to do the job - especially in the face of inflight emergencies and air-rage passengers.
Whilst I'm sure Nav 1 is a great cabin crew member, with all the relevant experience, let's remember that the more experience the better.
Darren - if you are still serious about becoming cabin crew, a good organisation in the UK is Heathrow Personnell (www.heathrowpersonnel.co.uk). They're very helpful in obtaining recruitment information for whatever airline you'd like to join.
Different airlines have different minimums (some have different minimum ages 20/21). There's a whole range of airlines you can join, with most having their hiring info. on the web.
Good luck with all your future endeavours!