How often do we ask ourselves.... Was my ancestor old enough to have witnessed this document? To have purchased land? Joined the local militia? Married the girl next door? The answers to these and other such questions are often as simple as determining the laws in effect at that particular time and place (if your ancestor actually followed the letter of the law, of course!).
Although federal and state statutes and session laws, both current and historical, are fairly easy to access--and often can be found online, locating a specific statutory law in effect at a certain period and place can be a little difficult. This guide to Finding Historical U.S. Statutes Online walks you through the basic steps, using the example question "What was the minimum age for an 1855 marriage in North Carolina without parental consent?"