Question: Why can't the BYU boys go on a mission after they finish school? Also, for the girls, they usually go after they are 21. Is that because they are considered to be an old maid and not likely to get married, so they may as well be on a mission?
They changed the age, so most girls are now going at 19.
As for why the guys don't go later, it's a lot nicer to get in a year of school, get used to being away from home, then go on the mission. Plus, when you get back, you have three straight years of running with the team and can build off of that later as opposed to spending four years with the team, then sucking it up and never getting close to what you were before going on the mission. They can go until they turn 27, but it just makes sense to go when you're 18.