Marriage is a beautiful thing to happen to anyone especially when the two persons involved are in love with each other and have a level of understanding of each other. Nobody wishes to marry late but circumstances may occur, either one is too busy to realize that he/she needs a partner or one was not getting the love they wanted and deserved.
Marriage is not something anyone should rush into as one must be sure to want to spend forever with each other before having a thought of marriage. Believe me that's another journey of life, not to mention when you start having babies and then bills to settle and still be there for the family.
Marriage is defined as the legally or formally recognized union of two people as partners in a personal relationship (historically) and in some jurisdictions specifically a union between a man and a woman. 
Well, I would say marriage is the union between two imperfect beings coming together as one with understanding and love towards each other.
Causes of Early Marriage
Poverty: This is one of the major causes which affects the lives of many young girls, especially here in the northern part of Nigeria by not allowing them to enjoy their teenage and youthful days. This applies to parents who give their children out in marriage to wealthy merchants to cope with their debts and escape their miserable state. Here, you see a girl of 12 years of age being given out for marriage and one would wonder what a girl of 12 years can offer a man who is old enough to be her father or grandfather.
Pressure (family pressure): This is applied due to competition between two families whose daughter marries at an early age. But from the Nigerians’ point of view, the parents believe that as a girl the moment you are done with your tertiary institutions, the next thing should be you bringing home the husband you want to marry. Or rather you hear bashes from parents saying "you are too old to be in their house that you ought to be married by now" and this does not affect the females alone, it also affects the male child. Pressure can also come from friends where a group of married friends tends to discriminate against the one who is yet to be married.
Unwanted Pregnancy: After poverty this is another very rampant cause of early marriage where parents whose daughters get pregnant, would be forced to marry the one who got her pregnant. This is very much rampant here in Nigeria. Some families who do not support abortion and do not also support giving birth out of wedlock.
Advantages of Early Marriage
Developing Together: It is said that when two young couples marry, they tend to grow and explore each other's character and above all discover new personalities for a longer period of time in their lifetime. Some will say "early marriage can be a great path for both partners to have a unique chance to grow from youth into adulthood side by side".
Sharing Habits: As newly married couples, it is understood that, as both of them tend to grow together, they also tend to grow some habits together. Getting to have some similar habits together is much better as they share similar interests instead of one person trying to change the habits of another.
Long Way To Go: This is best explained as one would say " building a life from scratch is one of the most impressive benefits of an early marriage". Thus having to enjoy crazy moments together, get to travel together, earn new experiences and not be in a hurry to have kids. However, they've got enough time to plan everything.
Plenty of Time to Divorce and Remarry: Now, how would one even think of a divorce? Well I get that people grow out of feelings for each other and would love to go separate ways via a divorce. This defines the reason for early marriage as it best explains that one can always remarry as they've still got years to remarry, especially those within their early 20's.
Safer Pregnancy: The couples have lower risks and chances of having miscarriages, as a woman due to her biological age-related changes. This applies to the women within the age range of 24-26. It is said that women of that age range (24-26) tend to have higher chances of carrying a healthy baby .
Disadvantages of early marriage
Having discussed the advantages, we still now discuss the disadvantages.
- Lack of compromising abilities in either partner.
- In the case of arranged marriages, sexual relations can be strenuous.
- Education tends to take a back seat as they now focus shift on financial stability and family planning. 
However, there is no guarantee that early marriage will make a couple happy and last forever as there is also no guarantee that it will not end in divorce.
In general essence, no one knows whose marriage would be a wonderful union. Therefore, we can't give one answer as to whether early marriage is good or bad, as everything depends on the couples, their willingness to listen to each other, find a compromise and preserve their feelings throughout the years.
I would rather say, for some people, it becomes the most amazing event of their lives, and others can also turn into major disappointments, as marriage on its own is another journey of life between two people who are ready to stay committed to each other.
 Oxford Languages
 https://www.legit.ng/(advantages of early marriage gotten from Legit newspaper)
 https://www.onlymyhealth.com/advantages-and-disadvantages-early-marriage-1309754987(disadvantages of early marriage written by Adith Dasgupta)
 Life Experiences