pallan: BA not accepting predicted grades isn't a new thing. It has been like that the last two years as well, although they have gone out of their way to make it abundantly clear on their website this time around.
But some 18 year olds will have A-Levels at the time of applying. And remember, it's not only open to English candidates. In Scotland it's normal to get Higher results while 18-years-old and be perfectly in time to apply for the programme.
Besides, from BA's point of view there may well be legal implications if they tried to exclude 18-year-olds from their recruitment. It might be construed as age discrimination.