Unless you left it without a belt on, then even the time it was sorn it should have been swapped twice.

My car isn't mapped, for crying out loud. And there's nothing wrong with my driving style, TY752 gearboxes are weak as fcuk. The TY754 box in my car has been in since 2008, broke the centre diff in that last year. :D

Changing a timing belt twice on a car that has done 125 miles in 6 years and stored in a heated garage, really, I just inspect it to see if it has perished which it hadn't so why change it
As for my car being neglected, you didn't seem to think that when you commented on it a while back, quit the opposite.
We all know the model Impreza you have suffer from weak gearboxes but to go through 4 in 150k seems rather a lot to me.
Like I said my car has had 1 timing belt change from new and isn't neglected as stated by yourself.
If you want to believe that a car that has been remapped and modded will last longer and have no long term affect on the life of the car without spending extra money on a regular basis then that is your prerogative but from my experience of modding and chipping cars Renault 5, cosworths chipping modding and running extreme BHP means spending lots of extra cash and far more maintenance than a car that is at factory settings

The belt will still degrade with age wherever he stores it.

Well mine didn't, I must have been so lucky,lol
So subaru give you a guide line of when to change your timing belt, lets say 50k, so we think at 51k the belt will go. do you not think that the car manufactures factor in some sort of safety so the belt is good for a lot more than they say and get us to change them more often than really needed.
How many people have had belts go on their Impreza, not that many I would guess or a lot less than the troubles people have had with the weak gear boxes and the 3rd pot on classics getting starved of fuel
Forgot to say, do timing belts come with a use by date or best before date?
Car parts are often stored for a long time before going to the shelf so if a belt is stored for 2 years does that mean it has 2 years less of a life when it is placed on the car?