As @MrItty is taking great pains to say, correctly, that there is no “one size fits all” response to this question, it is not “always okay” or “never okay” for these two people to have a sexual relationship. In some states (and other legal jurisdictions, including other countries) 16 may be a legal age of consent. In some others it is not.
But the OP is very adamant about stressing that this is NOT for a sexual relationship. In that case, it’s still not correct to state that “anything goes” if it’s not a sexual relationship.
Part of the reason is that we can’t all seem to agree on what a “sexual relationship” is (that is, where it starts), and part of the reason is that, again in many places where 16 is below the age of consent (or the age of “majority”, that is, the age when a person is a legal adult), then the child has to live with parents who themselves may say what is and is not okay for various types of relationships, including “just friends”.
GAs to @MrItty.