For several years now it's not been a rule about people posting their age. It was removed from the rules years ago along with a lot of other things to make these forums a more user friendly place.
No one can be found by posting their age. So it's upto each and everyone of us if we feel the need to share that then it's fine and fine with Jagex. Only time it would cause issues is if anyone posted they were under 13. Then it's dealt with on a case by case basis and refered to Jagex for review.
No action was needed on the thread you were concerned about as it's within the current rules.
@Archaeox - I don't really see the harm in leaving it there, as we've had a few RuneScape-adjacent topics that users don't really want to post in General in Off-Topic over the years and it's been fine. I understand where you're coming from, but as the topic's probably not going to last much longer anyway according to OP, I'm not super concerned about its placement.
Community Manager for Jagex