What "age of consent" actually means
It's the minimum age at which the law considers someone capable of legally consenting to sexual activity. Below that age, the law doesn't recognize consent as valid regardless of what was actually said or agreed to between the people involved — the legal question isn't about whether someone said yes, it's about whether they were old enough for that yes to count.
Why there's no single number
In the U.S., age of consent is set at the state level, not federally, and generally falls somewhere between 16 and 18 depending on the state — there's no one number that applies nationwide. Internationally it varies even more widely by country. Assuming your state's number (or a number you've seen quoted online) applies somewhere else is a common and genuinely risky mistake.
Close-in-age exemptions exist, but vary a lot
Some states have "close-in-age" exemptions — sometimes called Romeo and Juliet laws — that apply when both people are near the age threshold with a small gap between them. What counts as a qualifying gap, and whether an exemption exists at all, differs significantly by state. This is genuinely not something to assume covers a specific situation without checking the actual statute.
Why this is more than a legal technicality
The age threshold exists because the law recognizes that capacity to meaningfully consent — understanding what's being agreed to and having the power to say no without consequence — develops with age and isn't just about physical maturity. Power dynamics, including significant age gaps or positions of authority (a teacher, coach, or employer, for example), can affect the legal picture separately from age alone in many jurisdictions.
Where to actually check
Your state's official legislature or attorney general's website is the most reliable source for the exact current law where you live — statutes get updated, and a number quoted in an old article or forum post isn't guaranteed to still be accurate. National organizations like RAINN maintain resources on state-by-state sexual assault and consent laws that are worth cross-referencing if you want a starting point before going to the primary source.