Is double eyelid surgery preventative for sagging?

Idj,While results for double eyelid surgery done at a younger age can last longer than if done on a 50 year old, the skin will still eventually begin to sag.  As we age, the collagen fibers begin to slowly breakdown.  This has the effect of sagging of the skin.  Surgery only removes the excess sagging skin, however, a more effective way to treat this would be to combine surgery with skin laser and to continue with skin laser regularly.  This should help rebuild collagen and maintain elasticity.  Hope this helps.Hugo

As a general rule, the younger the patient, the easier it is to complete the procedure; as an added benefit, any redundant skin can also be removed. Compared to the Caucasian eyelid, the Asian mono eyelid has slightly more redundant skin. Furthermore, Asian people with mono eyelids will experience sagging around their eyes earlier than Caucasian people of the same age. 
If one decides to keep a mono eyelid until one is older, there are two procedures that can help a sagging appearance. One procedure is double eyelid surgery and the other is an underbrow eyelid lift procedure that is recommended because the former can appear quite unnatural on certain individuals. To further explain, the previously mentioned individuals are those who have thicker skin around their eyelids or have experienced sun-damage to their eyelid skin. 
I cannot say definitively but it is most likely the case that the men in your family with the unnatural appearance post double eyelid surgery would have either thicker eyelid skin or have suffered sun-damage. 
Sincerely, 
Dr. Andrew K. Choi

That's a tough one.  Believe it or not it is often possible to have an Asian Blepharoplasty aka Asian Eyelid Surgery without adding a crease.  If you want a small crease, that may be possible.  If you could provide some photos that would help.
While it is very true that surgery (of all kinds) earlier in life tends to look better and more "natural" than much later in life I wouldn't say you have to be in a huge rush.
surgery over the next 3-5 years should work.  However, if you wait 10 or more years, sure, your outcome will be less predictable and it will take you longer to look natural.OH! BTW, if you like your monolids do your best to keep that look.  Everybody else can go. . .you know.
Chase Lay MD
Asian eyelid surgery specialist