i think it might have something to do with litigation.
Wikipedia says that they have been sued by actors that got their age revealed on the message board https://en.wikipedia.org/wiki/IMDb#Litigation
And now California has a new anti-ageism law; wikipedia says "The bill forces IMDb to comply with requests by the actors featured on its pages to remove information about their age"
so they might have decided to close the message board for fear of litigation.
Section 230 of the Communications Decency Act protects services from being responsible for what users post:
"No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider"
They may not be responsible, but if they're asked to take it down via DMCA request or a court order, they have to either comply or go to court. Obviously if it's other user's content, it's just easier to comply. Once these start coming in more frequently and adding up, the maintenance costs of that user contributed content add up as well.
Although it's widely used for general censorship, the DCMA is supposed to only cover copyright issues. So the parent comment's concern about "anti-ageism" posts would not be a DCMA issue.
But that undoubtedly cost IMDb money. Litigation on its own can be enough to sway the risk/benefit scale, even if the civil complaint is entirely baseless.
For one, complying with the law is pretty important. For another, when your livelihood is based on (to a non-trivial extent) your physical characteristics and appearance, having your age revealed, and possibly aging you out of more glamorous roles, is an important protection to ensure.
I have no idea why this was down voted, it seems like it was a calculated decision. Litigation can destroy a company (just ask Gawker). Being responsible, legally with consequences, for monitoring the entirety of your site for age disclosure is expensive. Not only to do it, but the cost of litigation for when you miss one. That's probably a big part of the calculation.
Someone suggested that it's because moderating is expensive, which is true, but IMDB has been moderating their site for a long time. I don't think the cost of standard moderation was a big part in the calculation, simply because they have been doing it successfully for years. I think they are also aware of the value of their message boards simply by tracking page hits.
Having said that, public figures are excluded from privacies generally granted to the public. There are plenty of public records of everyone that discloses their age, even though they are not a public figure. To pass a law requiring IMDB remove age disclosure with consequences is very bad law. Not only that, the people that are the "victims" have the financial ability to litigate. It's even worse if other publications like tabloids are excluded from this requirement.
Will the law prevent age information on wikipedia articles? If i google an actors name and age first there is Googles inline information and then links to Wikipedia.
And now California has a new anti-ageism law; wikipedia says "The bill forces IMDb to comply with requests by the actors featured on its pages to remove information about their age"
so they might have decided to close the message board for fear of litigation.