>Just forget all the nonsense the homegrown preachers on this forum
>harp on about.
Donut, be nice :)
There are different varieties of English and often, it does depend which variety you happen to speak.
While I agree with your post and in fact I use (and a lot of people around me too) "will" and 'going to" interchangeably in most situations, it's quite possible that British English is more conservative and does make a clear distinction between these two. I'm too lazy to do the research though and frankly, don't care much.
I think most people here study/know British English.
"I'm going to be 21 next week" is correct in my variety of English (and probably in donut's too), but so is "I will be 21 next week".
I personally use "will" only when I want to emphasize the fact that something will definitely happen, e.g.
I 'will' go to Spain next year (it's a promise and I'll do everything in my power to make it happen)
versus
I 'am going to" go to'Spain next year (just a statement of the fact).