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.
 improved summary.
It's possible to express dissatisfaction with a title and to encourage better posting habits without being a jerk. I agree with your summary.
The problem isn't with HN being jerks to young people. The problem is with HN being jerks to anyone.
The main problem I think we have is that posts will continue to be 'link-baity' and some of those will be related to age. Each time one of these hits the front page the community will need to respond to it, and for the near future that response seems likely to be a reignition of this debate.
How do we educate 'drive-by-posters' who create an account simply to post an article (potentially with a link-bait title)? Is it up to moderators to change the title? Users to flag it? Should we just keep arguing about age in post titles?
I don't know the answer, but I think reasonable application of the posting guidelines is appropriate, and for me that means keeping the post title as relevant as possible. Most of the time, the age of the poster/developer is not relevant enough to be in the post title.