Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Answering your question, Sheikh Mustafa Umar, President of California Islamic University, states:
Any surgery for beautification is not allowed since it is considered to change the nature that Allah created people with. It is viewed as a type of mutilation of the body, even if that is the latest beauty fashion.
However, if there is a genuine need which cannot be accomplished another way, such as if someone’s nose was damaged, it is allowed to surgically restore it to the way it was, since this is not changing the nature of a person but restoring it back.
The same is the case when getting braces for your teeth: it is restoring the crooked or unaligned teeth to the way they should have been.
Likewise is the case with laser eye surgery: it restores your eyesight to what it should have been. So it will be allowed to get ptosis surgery if a competent doctor confirms that you have this condition.
Almighty Allah knows best.