Hallie

A Bar/Bat Mitzvah is an affirmation that someone has reached the age where they are obligated in Mitzvot.

It's definitely never too late to have a ceremony.

So, as long as you are over that age, the ceremony can be done at any time. While it is not required, the ceremony often involves doing a mitzvah that you couldn't do as a child, but that you can do now. This is why most people read from the Torah as part of their ceremony, because a child can't do that. Orthodox synagogues don't let women do this, but if you attend a Conservative or Reform synagogue, you should be allowed to read from the Torah as part of the ceremony if you want to.

If you have the ceremony on a holiday where there is a Megillah reading, another option is to read from the Megillah.