My prescription hasn't changed very much over the past 4 years - only an increase of .5d in each lense. The optician told me that growth spurts would generally decrease vision during puberty
Considering I'm at the end now (nearly 17 - not far away) then this means that I shouldn't grow any more, therefore vision should remain at it's present level until old age sets in - which is quite a bit away!
Andrew, I would recommend going for the class one if you can afford it, if your eventual goal is to become a commercial pilot. I do understand that the CAA are doing a Dick Turpin on it now, and charging something in the order of £400, but as a bullet to be bit (sic) at some stage it is one of the ones to chomp down on early.
A class one would be great just now - although I don't know if finances could reach just yet - my salary increases with age - so it won't be long until my salary grows a little and this will be possible.