Is it Kufr (disbelief) to mock or be disgusted by someone who has married an older woman? I am asking as I have seen this happen. Many muslims do this very often even though they know that the Prophet, sallallaahu ‘alayhi wa sallam, married Khadeejah, may Allaah be pleased with her.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

There is no doubt that it is prohibited to mock a Muslim and that it is a major sin. Ibn Hajar Al-Haytami said, "Major sin number 251: mocking and ridiculing a Muslim. Allah, The Exalted, says (what means): {O you who have believed, let not a people ridicule (another) people; perhaps they may be better than them; nor let women ridicule (other) women; perhaps they may be better than them.} [Quran 49:11] We have previously discussed the interpretation of this verse. Muslim scholars unanimously agreed on the prohibition of such an act." [Az-Zawaajir ‘an Iqtiraaf Al-Kabaa'ir]

Mocking a man for marrying an older woman falls under the category of prohibited ridicule and mockery. However, this act does not necessarily mean that this man has mocked the Prophet, sallallahu ‘alayhi wa sallam, and accordingly has committed Kufr (disbelief) on account of that. A Muslim is not accused of Kufr because of such an action.

It seems to us, based on this question and a past one, that you may be suffering from some form of Waswasah (obsessive doubts) regarding Kufr. So we advise you to beware of such Waswasah.

Allah knows best.