Would v line and cheek bone reduction give me saggy skin/ cause premature aging ?

Jawjaw contouring and cheek reduction will reduce the volume of your face. In general this would make your skin looser. However the amount of volume that is actually removed is not so significant that you should have a problem. If you are young and healthy, and an appropriate weight, we would certainly expect your skin to Contour to the new facial skeleton. These are procedures that I do many times each week and have for many years. The results you want should be safely achievable.

The amount of sagging skin after the Vline surgery depends on each patient.  Each of my patients wears a facial garment afterwards to help support the soft tissue and have it seal to the new bone.  I have found that in young patients with elastic skin this usually heals fine. I do offer patients the Renuvion (or Jplasma) skin tightening at the same time.  This is great for older patients, or young patients who have some skin laxity.  This can be done at the same time as Vline surgery, or after if the patient still wants tighter skin to define the new jawline. Of course, visit with a craniofacial surgeon to see what options best fit your face. 
Best of luck
Dr Rodman

In general, the more bones are resected, the more sagging may appear, especially if the patient is 30+ years old. 
In general there are 4 factors that influence the risk of getting saggy skin: 
- whether you have heavy fat on your cheeks
- whether you skin elasticity is low
There 2 depend on each individual skin type or body structure. 
- if you have to reduce the big amount of bone
- the desquamation area is wide 
These 2 depend on surgeon's skills and experience. 
When the cheekbone reduction is performed the flesh of the bone must be separated. It's called "desquamation". After separating the flesh surgeon performs bone reduction and fixation. The problem is that when the surgery is finished the flash should be attached to the original position. However if you have a lot of fat on the cheek and you skin elasticity is low the skin cannot hold the cheek fat in position, so it will lead to sagging.   In this case ID hospital analyzes each patient's skin condition, age and bone structure and if needed recommends Accusculpt laser or Buccal Fat Removal to remove excess fat and avoid sagging. 
With the right treatments and experienced surgeon there is no need to worry about saggy skin after the surgery. I hope it helps.