Faatimah was eighteen when she married ‘Ali
27-2-2017 | IslamWeb
Question:
When the Companions asked for Fatimah, the daughter of Prophet Muhammad, sallallaahu ‘alayhi wa sallam, in marriage, why was she considered too young? Was she only five? I am asking because Islamqa.info says that you can marry at six, but this seems to contradict that if she was six or older. Also, some of the Companions said not to marry your young to the old people for they love what you love. So how old was Fatimah, may Allaah be pleased with her, when they asked for her in marriage?
Answer:
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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
We do not know to whom you are referring in the question, as Faatimah 
was married to ‘Ali 
when she was eighteen years old. In his book At-Tabaqaat, Ibn Sa‘d 
said, “It was narrated on the authority of ‘Abdullah ibn Muhammad ibn ‘Umar ibn ‘Ali that his father said, ‘‘Ali ibn Abi Taalib married Faatimah, the daughter of the Messenger of Allah, sallallaahu ‘alayhi wa sallam, in Rajab, five months after the Prophet made Hijrah (emigration) to Madeenah. He consummated his marriage with her after he returned from the Battle of Badr, and she was eighteen years old when he consummated the marriage with her.’” [Tabaqaat Ibn Sa‘d]
When Muslim scholars addressed the issue of the young girl's marriage, they used the story of ‘Aa'ishah whom the Prophet 
married when she was six, and he consummated marriage with her when she was nine, as evidence. We have previously clarified in some of our fatwas that there is no religious impediment to marriage at such an age, and we refute the misconceptions raised by some people in this regard. Please refer to our fatwas: 88089 and 81505.
There is no religious impediment to the age difference between the spouses, whether the wife is older than the husband or vice versa. This is evidenced by the Prophet's marriage to ‘Aa’ishah, as we mentioned, and his previous marriage to Khadeejah who was older than him.
The words which you referred to were said by ‘Umar ibn Al-Khattaab as he said, “Do not make your women marry the ugly, unsightly man, for they love for themselves what you love for yourselves.” It was cited by As-Saffaareeni in his book Ghitha’ Al-Albaab. It is clearly related to the outward appearance and looks, not the age. ‘Ali ibn Abi Taalib gave his daughter Umm Kulthoom in marriage to ‘Umar when she was at a young age. Whoever marries such a great man is indeed quite fortunate; many women wish to have husbands such as him.
Allah knows best.