Guys have it easy - the answer is "Yes."
Girls have it much tougher - the answer is a clear, unequivocal "Maybe."
Here is how President Monson described it last year in October Conference:
"We affirm that missionary work is a priesthood duty—and we encourage all young men who are worthy and who are physically able and mentally capable to respond to the call to serve. Many young women also serve, but they are not under the same mandate to serve as are the young men. We assure the young sisters of the Church, however, that they make a valuable contribution as missionaries, and we welcome their service.”
That's why the potential Sister Missionaries have it tougher - it is not a "mandate," or a "priesthood duty." It is a choice they have to figure out for themselves.