Void marriages don't legally exist in the first place -- they're against the law in most states. They include incestuous marriages, such as if you married someone closely related to you by blood. They also include bigamous or polygamous marriages, where either you or your spouse was married to someone else at the time of your wedding, whether or not you were aware of it. You can usually annul these marriages as long as both you and your spouse are living, because they were never valid at all.
Voidable marriages are legal, but so flawed that the law says they shouldn't be recognized. If your spouse tricked or forced you into marrying, if you or your spouse didn't understand what you were doing at the time due to mental incapacitation, or if one of you is impotent, these are all voidable marriages and annullable in most states. However, statutes of limitation for annulling voidable marriages can range from a few months to several years, depending on your state's laws. The time frame usually begins with the date you learned about the circumstance that makes your marriage voidable. In Colorado, you have only six months if your spouse tricked or forced you into the marriage, but a year if your spouse is impotent. California allows you four years in these circumstances, but you have only two years in Ohio to file for annulment of a voidable marriage. If you or your spouse was not of sound mind at the time you married, some states allow you to file for annulment until one of you dies.
When minors marry under the age of consent and without parental or court permission, these unions fall into a category all their own in some states. Statutes of limitation often depend on the age of the parties at the time they request an annulment. Although Colorado has a two-year limit on when you can annul such a marriage, other states draw the line after the minors reach the age of majority. For example, in California and Ohio, they have two years after reaching the age of 18, although their parents can have their marriage annulled before they turn 18.
Some states have statutes of limitation for annulments regardless of your grounds. For example, in Virginia, you must file within two years, and -- in the case of a voidable marriage -- you cannot have lived together after you learned of the circumstance that makes your marriage voidable.
In most states, it's not enough to simply allege your grounds for annulment -- you have to prove them to the satisfaction of the court as well. This can take some time and can affect your statute of limitations. For example, if you claim that your spouse was married at the time he married you, you must usually provide a copy of the marriage certificate from his previous marriage, prove that his other spouse is still alive, and prove that they were never divorced. Tracking down all this documentation may take months. Speak with an attorney in your state to learn the deadline for the type of annulment you think you qualify for, and begin gathering documentation as soon as possible so you have ample time to prepare your case for the court.