Are they allowed to simply mention that they make mature content on their profile as long as they don't provide any links to the content?
As long as it's kept vague and just stating what you do and not advertising what you do, it should be fine. If in doubt, send the exact wording to a staff member before you post it publicly. We'll be happy to sign off on stuff.
Example:
"I'm a writer, I mostly write mature stories." <- Fine
"I'm a writer, I love writing sexy adult stories and would love to share them with you~ " <- Not fine
If they are approached by another forum member age 18 or order over personal messager about wanting to see their adult content, are they allowed to send that forum member a link over personal messager?
Anything outside forum turf is generally fine. However, if for whatever reason this sort of thing causes issues between you and another forum member, and those issues are brought to the staff's attention for us to resolve, it may very well be relevant.
Hope that clarifies!