StackExchange site solves this problem by gradually increasing user powers with their karma. Then even if the old guard spends less time online, their actions are more visible.

On the other hand, there are not as many "actions" one can do on LW. And we probably wouldn't want to limit things like "announcing a new meetup" to old users.

Here is a list of possible LW actions that could require some karma threshold:

• upvoting comments
• downvoting comments
• publishing articles
• upvoting articles
• downvoting articles
• editing wiki
• commenting in troll threads
• moving articles between Discussion and Main

Beyond that, I don't know. Perhaps users with huge karma could get ×2 or ×3 multipliers when voting, but more than that would probably be a bad idea.