Try to get some status on NW or CO before buying the lifetime pass. There is a big difference in price as you go up the elite ladder. Maybe buy a one year membership now. If you're doing enough flying to make a membership worthwhile you'll probably attain some elite level. Then buy the lifetime pass.

Of course, the younger you are the more sense a lifetime pass makes. Just doing a rough comparison of buying annual memberships versus a lifetime, you would need to use the lifetime membership at least 16 years to break even (assuming a 5% time value of money). You're protected against future price increases with the lifetime membership but you risk an airline bankruptcy or a future "enhancement" of club amenities or locations or alliances that may make the membership less valuable to you.