I posted something on the thread that started this whole dance a day or so ago, however it probably merits further expansion.
Most arguments that are 'pro' age disclosure boil down to "We should treat younger members of the community differently". This comes in many forms, like how we should be supportive, how we should mentor the next generation, and how teenagers are often insecure.
I agree with them, at least in general.
Arguments 'against' age disclosure have a few flavours; some say that we should treat posts based on merit, that we should not discriminate based on age, but mostly it comes down to "Posts with the age in the title are 'gaming' the system (purposefully or not)" and the meta-argument that such posts encourage talking about community instead of the post.
I agree with this side as well.
The thing is, placing your age in the title of your post is link-baity. It may be unintentional or it may be coolly calculated, but in most cases if you remove that piece of information the post title is entirely uninteresting.
So we have two types of posts to consider. The first are those posts where the age of the person involved contributes to how interesting the post is to HN. A completely fabricated example: "12 year-old entrepreneur youngest funded by YC".
The second type of post are those where the age of the person involved contributes to how the HN community responds to the post. A young developer's first app would fall into this category.
In the first case, I believe this is useful information to have in the post title. In the second this is useful information, however I do not believe it is useful in the post title. For these posts, I would much rather see the age included as a comment on the post.
In any post that use of age in the title gets called out for being 'link-baity', 'manipulative' or 'gaming the system' the main counterargument is that knowing the age helps the community know how to respond to the post. The age can be removed from the post title, and placed in the comments, addressing both major concerns.
[edit] improved summary.