Is being offended a harm? If so, should it be illegal?
No. And no.
That would move the discussion from "offense" to "harm" so that a rational discussion could be had.
No, it simply miscasts offense in an attempt to gain an unjustified rhetorical handle on it.
Offense is like a water balloon thrown from a bridge at a passing car.
No. It isn't. When someone messes with your wallet, your person, your reputation, your family in like manner, or your property, you have been interfered with. Actions designed to remedy the interference can now be put on the table. This is a very sane way to look at these matters, and it is generally what anyone seriously considering them will come up with. From it, we derive that any such action constitutes violation of the principle that "your right to swing your fist ends at anyone else's nose", or in other words such action is "swinging one's fist where it impacts the other person's nose" and so we don't accept it as valid action when considering these issues.
The water balloon constitutes physical interference with your property, your path, and your ability to drive in a safe manner, thereby additionally and (further) irresponsibly constituting risk to yet others via potential secondary and tertiary effects. Your suggestion is not the same, or even similar to, someone cracking a dick joke.
Does it matter if the "offensive language" is an adult trying to talk a mentally ill minor into suicide?
This is not "offense." This is incitement and inappropriate exercise of power. You are moving the goalposts quite a distance here.
"Offense" is seeing or hearing something that you don't approve of, and undertaking, at a minimum, a line of thought that criticizes that thing. Offense may often further extend to a verbal or written reaction, but its root remains in your own thought processes, for which you, and only you, are responsible.
If others are made responsible for what you think, then it follows that everyone is responsible for everything everyone else thinks in an unending causative loop, and as virtually everything offends someone, life would be constrained to living in an isolated environment so no one could see, hear or otherwise be exposed to you -- as your speech, appearance, actions, or even your very presence might very well be offensive to them.
It is perfectly legitimate to argue that something you find offensive, should also be found offensive by others; even so, no one has the obligation to agree with you, or even to pay you any attention. But when you impose control of others based on your perception of offense -- legislation, rule making -- now we're back to behavior that can only be valid on property you own or otherwise control (rent, have custody of, etc.)
On the other hand, tolerance, that thing we offhandedly characterize as "live and let live", is a social practice that leads to people generally not interfering with others, or even confronting them with argument. Highly recommended. It's very respectful of the personal agency of others. Note that this is not advice for discussion; it simply applies to venues and happenings that you were not involved with in the first place. There's nothing wrong with amiable discussion of issues that concern you, of course, as long as the other party(s) wish to engage.
