There are a few important things to conider here. If you were to apply as airman, the time you would spend applying, training and then working up to the rank which makes you eligibile to apply for a comission would take you way past 21 and there is no guarentee an Officer application later on would work out.
Also, if you wait a year, provising the role is still open, you can apply as an Officer. If this doesn't work out, you can just go in as airman. It seems to me that this would make more sense as you're giving yourself two chance here, rather than potentially only one from the first option (joining as airman now).
I would do something worthwhile in the meantime and give yourself a year of preparation and then apply at 21. You can always register before then, and then appply as soon as you're 21. By 'something worthwhile', I baically just mean boost your CV if it isn't strong enough already. If anything is worth doing, then it's worth overdoing. Get involved in ome local leadership activities and things like that.
It up to you what you do but I really do think this year would be better spent preparing for an officer application next year.