I want to chime in and add that there is a huge advantage to marrying young enough – while not “completely mature”… growing a bit together can do wonders for a couple to feel really connected and “part of a whole”.
While I am not going to claim that most chassidish marriages are perfect, I’ve seen enough to realize that NOT yet being too “set in one’s ways” is an enormous advantage to a marriage.
That being said, there has to be some level of maturity. Which CANNOT be measured in numbers/age. Some are mature enough by 17 – and some are not even at age 30.
Same for boys and girls. (Although as a general rule of thumb, yes, girls mature quicker than boys…)
One more thing: I agree with the commenter above who said its the MIDDOS that count for marriage readiness, and although being more mature means probably having worked on one’s middos longer… it doesn’t HAVE to be. If they both FEEL they are ready to take on the responsibility of marriage and have good middos, I would stake a bet that such a marriage will succeed, even if they are considered young and a tad immature.