For people who want to avoid old aircraft, it's better to look at the equipment used by the competing airlines on the specific route you plan to take. For example, if someone avoided NWA because of the old DC-9's but was traveling to Europe, they would miss out on the newest transatlantic equipment, their new A330's.
Personally, I consider maintenance to be much more important than aircraft age. I'd be much more likely to avoid flying on a newer plane with suspect maintenance than an older one with excellent maintenance. I'm always interested to hear anything about poor airline maintenance.
Last week I spoke with a recently retired AA
pilot. He spoke very highly of their maintenance.