Is 18 Years Old Too Young for Eyelid Surgery?

Hi Alexa
She may or may not be too young to make this decision. Certainly she should make this decision with the help and support of her parents. She may very well have a functional issue. If the droopiness of the upper eyelid is due to the eyelid margin (where the eyelashes are) resting low on the eye, this condition is referred to as upper eyelid ptosis. If your are referring to the upper eyelid fold, this needs to be more carefully assessed.
I would strongly recommend assessment by an oculoplastic surgeon. This is a board certified ophthalmologist who has completed a fellowship training in eye plastic surgery. The American Society for Ophthalmic Plastic and Reconstructive Surgery maintains a website that can help you locate a well qualified surgeon in your area (www.ASOPRS.org).
As these are very specialized issues, it is best to see a fellowship trained eye plastic surgeon rather than a generalist like a board certified general plastic surgeon or a facial plastic surgeon. And yes, the results of surgery need to last a lifetime.

It really depends what we are talking about. If she really has excess skin that can be improved by removing it, then so be it. But she would need to be examined to determine the proper diagnosis and then the treatment plan would be based on the problem at hand.
Age does not always play a role.
Good luck.