Wiping the ears in wudoo’ is something that the Prophet (blessings and peace of Allah be upon him) always did, and the scholars differed concerning it as to whether it is obligatory or Sunnah. Some of them said that it is obligatory, as is the view of the Hanbalis, because of the report narrated by Ibn Maajah from ‘Abd-Allah ibn Zayd (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The ears are part of the head.” There is some difference of opinion as to whether this hadeeth is saheeh; it was classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
If the ears are part of the head, then wiping them in wudoo’ is obligatory, as is wiping the head.
But the majority are of the view that wiping the ears is Sunnah and mustahabb, but is not obligatory.
See: al-Mawsoo‘ah al-Hadeethiyyah, 43/364
What is narrated from Imam Ahmad (may Allah have mercy on him) is that if a person omits to wipe his ears, his wudoo’ is still valid.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni, 1/90: The ears are part of the head, so by analogy our view is that it is obligatory to wipe them when wiping the head. Al-Khallaal said: All of them narrated from Abu ‘Abd-Allah concerning the one who does not wipe them, either deliberately or out of forgetfulness, that it is still valid. That is because they belong to the head, but we do not understand that the word head refers to the ears too, and they do not resemble any part of the head. Hence wiping them does not count as wiping the head according to those who say that wiping part of the head is sufficient. But it is better to wipe them when wiping the head, because the Prophet (blessings and peace of Allah be upon him) wiped them when he wiped his head. Al-Rubayyi‘ narrated that she saw the Prophet (Allah be upon him) wipe his head, front and back, and his temples and ears once. Ibn ‘Abbaas (may Allah be pleased with him) narrated that the Prophet (blessings and peace of Allah be upon him) wiped his head and his ears, inside and out. Al-Tirmidhi said: The hadeeth of Ibn ‘Abbaas and the hadeeth of al-Rubayyi‘ are both saheeh. End quote.
Based on that, if a person forgets to wipe his ears, he is fine and his wudoo’ is valid.
And Allah knows best.