If you want to avoid a hassle, I've found a bunch of sites that have something along the lines of "If you're using this site, we assume you're [13/16/18]+, plea

If you want to avoid a hassle, I've found a bunch of sites that have something along the lines of "If you're using this site, we assume you're [13/16/18]+, please don't use this if you're under [13/16/18]" buried in their terms of service.