No one is able to say whether you will or won't be able to get in based on your age, it all depends on the airline, the recruiters, your maturity etc.
If you meet the requirements for an airline in your country, apply for it! You've got nothing to lose!
In Australia at least, airlines can't discriminate based on age, but I think that some would be wary that, as an 18 year old who has just finished school, you may not have yet had the life experiences, and therefore gained the maturity, to for a job which is a lot of work & where a lot of responsibility is in your hands.
But by all means, apply for any carriers which you are eligable for! If you don't have any luck, perhaps work or study for a couple of years, then try again
