These terms are often used interchangeably, and they both seem to have a relatively negative connotation in today's society. However, I would argue that there is quite a large difference between marrying early and marrying young, and only one of these is a bad thing.
Getting married early is not a good thing, regardless of your age. You can be 20 and be getting married too early—or be 35 and be getting married too early. What do I mean by this?
Marrying early involves rushing into a proposal, not taking enough time to learn your partner, and being unprepared for the reality of marriage.
While timing is subjective and dependent on each couple, it is never healthy for either party to feel rushed. Oftentimes, I will hear women talk about wanting to get married at a specific time they have planned (right after college, a year before I want kids, etc,) and some go to extreme lengths rushing to ensure they are wed by that time. Albeit less often, the same can go for men who feel they've been bachelors for too long (i.e. "The Bachelor"?!?). Rushing into anything serious in life is almost always a recipe for disaster, especially with something as important as marriage.
A byproduct of rushing this process can also result in not truly knowing your partner as well as you should. You will be living with this person the rest of your life... at least ideally. Divorce rates have been at an all time high in the past decade in the United States, with the most common reason being "irreconcilable differences." Much of this probably has to do with individuals suddenly noting traits, attitudes, and quirks they don't like in their partner that they overlooked in the pursuit of marriage.
The reality of marriage is not all sunshine and flowers; more often than not, it can be quite difficult. By rushing into such a sacred commitment, people who get married too early often find themselves dissatisfied and wishing they had done something differently.
Now, let's talk about getting married young.
Two girls I went to high school with are married, a few others are engaged, and I myself am in a long-term relationship (all 19-21). I can't attest as to whether they got married early, but I sure do get tired of hearing people say they got married too young. If you are confident in your partner's ability to be a good husband/wife, have prepared yourself financially, and feel as though the Lord is guiding you to that person, what difference does it make how many years you've been on this earth? If you have been dating an individual a considerable amount of time and feel prepared to take that next step of commitment, there should be no place for judgment based upon age.
While these two terms have been used interchangeably for a while, I think it is time to reconsider what each means. Getting married too early is what often results in broken families, dissatisfied marriages, and divorce. That is what should have a negative connotation.
Getting married young simply means you were lucky enough to find "the one whom your soul loves" as Song of Solomon says, at a young age. All in all, there's nothing negative about that at all.