I would disagree with raising the year count because that's effectively limiting it to people who joined before X date instead of it being what it is actually called; people who been here for X years.
Simply add new time milestones as time passes. Don't take away old ones.
What I do think is that there should also be a minimum post count requirement. Maybe something averaged to 100 posts for year? So, to get a 5 year one (if it exists), you'd need 500 posts, and for 10 years, you'd need 1,000 posts, and so on.
I say this as someone with the 10 year one who didn't become active until recently. I originally joined because one of your members found me on the Sims 3 forums, haha, so I posted for some application they had and never intended to stay. Came back when I had a problem, "oh, I have an account already" and sort of stayed., mostly for game discussion.
I don't think people should be limited from these because they didn't join/weren't active in the 2000s. At the same time, 100 posts per year requirement along with the years themselves sounds fair. That way people who never truly stay active don't get it, but have a chance to get it if they return. That feels like the fairest compromise to me.
Edit: As for where they are placed, I don't care. Where they are, under profile, hidden in user profile, it's all the same to me.