so sometimes when a manufacturer says lifetime, it doesnt mean the lifetime of the person, it is the lifetime of the product, which may be 10 years or so.

I would suspect that if you tried to submit a warranty claim on a bicycle that was 20 years old they would say that product has reached end of service life and is no longer supported. At the very least you would need to prove that the bicycle was never used in manor which would negate the warranty such as taking it off jumps or being worked on by a person who was not an authorized trek dealer.

with all that said I would not be concerned about your frame failing so long as you didnt leave it outside in bad weather for years or dent the tubes by large impacts.

so like zacster I also have bonded alum frame from the 90's that has taken a holy god lot of abuse over the years and it still holding together just fine. Also as he said, its the ride quality of alum that commuters dont care for on asphalt not the durability.

One final thought, if your frame need to be replaced for any reason alum frames are cheap and easy to come by.