Interestingly, these laws vary A LOT by state-to-state,
and even moreso by country-to-country.
I researched this a few years ago (pissing contetest on the the internet w/ some self-righteous folks).
Many states are as low as 16.
Some european countries are as low as 14.
Personally I think 18 is uber-conservative
(or your typical CA self-righteous Liberal)
I've never, ever been an advocate of the idea that somehow 18 should be the cut-off point of age of consent.
Ages of consent in North America - Wikipedia, the free encyclopedia
Ages of consent in Europe - Wikipedia, the free encyclopedia
To more specifically answer your question though,
(I am guessing you live in CA)
Yes you would get in trouble.
However, it would only be a misdemeanor.
The age of consent is 18, with a misdemeanor if the minor has 3 or fewer years of difference with the major, and potentially a felony if the major is more than 3 years older. It is worth emphasizing that unlike most other states, the close-in-age rule in California (3 years) do not provide an exception nor provide any defense; it merely lowers the crime to a misdemeanor. Under this law, two minors of the exact same age could both be prosecuted. Penalties increase if the minor is under 16 and the major is above 21 or if the minor is more than 3 years younger.