Don't sue the dating service that set the two of you up, in a context where it was clear the dating service wasn't screening members for age (and couldn't reasonably do so). That's basically the holding of Doe v. SexSearch from the Sixth Circuit.
And, no, an incautious statement on the site that "all persons within this site are 18+" isn't going to get the service on the hook. "[H]aving registered for the site himself, Doe knew that SexSearch merely required a user to check a box stating that he or she is at least eighteen, with no corroborating evidence required from the user and no attempt at verification made by SexSearch. As a result, there could be no justifiable reliance on the [18+ statement]." Sounds right to me.
Thanks to How Appealing for the pointer.