In Islam, one is allowed to get married once attaining physical and mental maturity. There are also other conditions such as mental soundness and the financial ability to provide for the spouse. If both sides are able to fulfil the conditions, they are allowed to get married.
In Islam, age barrier is not a concern. The first wife of Muhammad (SAW), Khadijah (RA) was 15 years older than he was, and this monogamous marriage lasted until her death about 15 years later. This dispels the cultural expectation that women should not marry younger men. His subsequent marriages were with women of different ages, showing that age is not a barrier to a marriage, whether older or younger. Additionally, it is noted that all but one of this wives (Aishah RA) were previously married, some of them with children from their previous marriages, demonstrating that divorce or widowhood should not affect a woman's marriage prospects.
However, you must be honest about your own fundamental information such as your age and health. We are not allowed to deceive our future spouses by pretending to be younger or older than we are, and this includes indirect deception such as wearing make up, dyeing our hair or wearing wigs which disguise our true age. All relevant facts, and not just the age, must be presented upfront, without any deception or trickery. For example if the person is battling a chronic disease, or has a hidden psychological issues, is in debt, or suffers from a transmittable disease, this must be disclosed before the marriage itself.
Today we are focused on the age of the spouse, rather than what that spouse has to offer us in terms of knowledge, quality behaviour and deen. Nowadays, when a man marries a woman half his age, it is mainly to solve his mid-life crisis and for a second lease at youth. It is not because he wants to guide her towards being a better Muslim or establish the deen.
Whatever the age, what is the point if we are married to someone who is hollow and has nothing to offer us apart from dunia pleasures and status? A marriage is a 24 hour commitment, and while you may have a few hours of daily pleasure in the early days, then what happens next? How much intimacy can you handle in a day, and is sex your main goal in a marriage?
Many of us reject good marriage prospects because of the age gap. A twenty year old woman might be repelled at the prospect of marrying a thirty year old man, and society encourages that thinking by making it seem that such an age gap is socially objectionable. Very rarely does a woman marry a younger man without being gossiped about. All these barriers do not exist in Islam and are imposed culturally.
On the other hand, there are now men and women who seek significantly older spouses for marriage purely to access the older person's wealth or social status. How many of these marriages are for the sake of enriching one's self development or knowledge?
There are other realities to consider. For example if you would like to start a family, then it is only sensible to pick a wife or a husband of a certain age group who is capable of parenthood. If you would like to marry someone younger than you, you have to consider whether the person is mature enough to be your spouse. If you marry someone significantly older than you are, then you have to be ready that he or she may not have the same energy levels, and might require extra care as they older. In addition, be prepared to accede to that person's life experience, for he or she would have had a head start in life. If marrying someone younger, then be prepared that the person would be at a different maturity level from you and may not see things with the same experienced eyes as yours.
Be that as it may, we are not encouraged to delay our marriages in Islam – unnecessary delays are one of the reasons for so many ongoing fitnah in this world. Because of certain material pressures, many people keep delaying the marriage until they have a good income and are able to provide quality housing and lifestyle to the spouse. In many societies, the men are expected to be millionaires first before they stand a chance of any good marriage prospect. By then, the person's lifetime habits are deeply ingrained, and it becomes difficult to adapt to new habits of living together. This can range from hygiene, neatness, preferences, or even methods of washing the dishes. The stubbornness will also increase because with age comes certain fixed behaviours and preferences which are hard to shake off. A younger person is malleable, and if both husband and wife are young, the advantage is that they can mature, learn and grow together.
When Muslims approach marriages, they lose Islam because they are willing to reject the Qur'an and Hadith for the sake of the marriage. They invent rules which are not there, and this is why we have social problems which never existed during the time of Rasulullah SAW. In case of doubt, reassess your original intention of marriage. If done for the sake of Allah, with the right intention, and in accordance with His guidelines, insha Allah the marriage will be successful.