Is it permissible for a married woman to undergo cosmetic surgeries to correct sagginess caused by aging, due to her being affected by her husband's gaze towards other women, and her desire to restore her youthful appearance, knowing that this is not to alter God's creation?

Cosmetic surgeries performed merely for adornment are not permissible. Those that are permissible are for removing a defect that disfigures one's appearance. Therefore, it is not permissible to proceed with this matter. Your justification, that your husband is attracted to women in the media, is not a valid excuse. Rather, you should address his attraction by beautifying yourself for him in ways that are permissible, and by advising him to stop looking at non-mahram women and warning him about the danger of gazing at what Allah has forbidden.