Hello. I have an 09 Forester Premium. I just had to have the rotors turned at 19,000 miles! This sounds a bit early, you think? There was a pulsating upon braking and alas the mechanic found they were warped.
Pulsating brakes doesn't necessarily mean they are warped. I actually had really bad runout on my original rotors somewhere around 15,000 miles, but wasn't experiencing the pulsation. I really don't know what it was. During the first year, it was mostly my wife's car until my old car got wrecked. I replaced the rotors with Brembos and bedded them in properly, and now I have 90,000 miles on the car on the second set of rotors.
Uneven brake pad deposits. Smooth braking action and proper bedding will help extend the time to which it happens. Additionally, regular brakes maintenance (just a good old sanding of the rotors and pads, and the reappliation of brake grease [after cleaning everything well first]) will help prolong the period as well. You will notice that the rotors will start looking like a section of a tree trunk, with different thickness lines at different radii.
Both over torquing and torquing the lug nuts in the incorrect sequence order can cause warped rotors. In fact, if you don't do any real hard driving and you are getting a brake shimmy, that is the likely cause.
Can the rotors get warped if you did alot of winter driving in the snow and let the snow get caked up inside the wheel? I heard somewhere that can cause a little warping especially if you dont get all the snow out while it is still hot and it cools down with the snow/ice and then freezes over night.
According to 09 manual, wear items such as brakes and clutches are covered for 3 yrs. or 36000 miles. My rear rotors are shot after only 10,000 miles!(deep gouges in both) Dealer 420 miles away so trying to weigh my options on getting them replaced under warranty.