Originally Posted by Salamandrine

Specifically, the steering tube will tend to bulge out after a long time, and then it's in danger of fatigue cracking. Can't be fixed other than by brazing in a new one. It's not the age, it's the miles, and probably the size of the rider and terrain the bike was ridden on. Honestly, 99.9% of people don't have to worry about it. Think 50,000+ miles. (see what i mean.... ) Just inspect the steer tube as it is very obvious when it sets in.

I pretty much agree with these opinions and observations:

Threadless Steerers and Headsets by Jobst Brandt