Presuming this was done where each person had fine control over the date, you may be right. However, suppose each person only had a single binary control, essentially "hide everyone newer than me."
Furthermore, rather than "newer than me" being defined by a join date specific to each user, instead it would be defined by the start date specific to a particular generation of users (the news.yc folks do seem to track mass user migrations). Visually (bars are new.yc join floods, time moves to the right):
time: |--------|---|------|-----|---...
group: 1 2 3 4 5
Joining early in group three, you would be able to view comments made by those in groups 1, 2, & 3, which would include people who joined after you in group 3 (your supposed peers).This would still provide each user with the ability to view the site more-or-less as it was upon first arrival, without actually allowing each user fine granularity in locking everyone out.
(for the record, such a feature isn't as useful as simply applying social & administrative pressure to keep quality up; I just wanted to mention an alternative implementation that doesn't turn into an arms race)