depends on their maturity level, if they are grown up enough, and love each other then there is no reason for them not to.
on the otherhand, if they are imature, pregnant and just doing it for the baby, NO..
of course, they should be over 16 and very mature for their ages, but everyone is different. they have to be able to support themselves financially and emotionally and have at least lived together for a year or two first..
honestly tho, there are very few who are mentally grown up enough to be married under 21 and last a lifetime.. it often falls apart within the first year. but like i said.. everyone is different :)