I'm 26yo female considering V Line surgery. How common is severe sagging, premature aging, & a double chin at my age? (Photos)

You are correct to assume that loss of bone support can result in the soft tissue symptoms that you have described. Typical v-line surgery, in which the full thickness of the bone is reduced along the entire jawline (vertical length removed) and the width of the chin reduced by a wedge t-shaped genioplasty, creates a substantial loss of bone volume/support to the lower face. While this approach produces the most dramatic change it can cause soft tissue problems if they do not fully contact down to the reduced bone. Usually the young age of the patient permits good soft tissue adaptation to the bone in most cases. But when soft tissue problems occur it is the direct result of a mismatch between the overlying soft tissue envelope and the volume/shape of the bone underneath it. While such soft tissue problems  do not always occur there is always a risk. Having seen and treated many v-line patients with these soft tissue problems, they are difficult to fully resolve. The alternative approach is to do a more limited type of v-line surgery in which the height of the jawline is preserved posteriorly but only its width is reduced. The chin may still be treated by a t-shaped genioplasty but its central wedge bone removal is kept more limited. 

Greetings. Thank you for your question. 
There is a possibility of sagging after facial bone contouring V line surgery  if your skin elasticity is low or a big amount of bone reduced. The sagging can be prevented by an Acculift laser, combined with a surgery. Also it's very important to get a surgery from an experience facial bone contouring surgeon because he / she decides the amount of bone that can be trimmed.
In general. patients over 30 get are tend to get more sagging. As I can see from your picture, you are a  very good candidate for V line surgery. A surgeon need to examine your facial bone structure and X ray face to face to give a more detailed answer. 
Best wishes, 
Park Sang Hoon, ID Hospital, South Korea