Howdy.
Frame life is related to how it's used. If I leave a carbon bike hanging in my basement for three years, it'll be fine three year later. If some 250lb weight lifter sprints everyday on any bike, he'll put it through stress cycles to fatigue it eventually.

The reason the different material have different recommendations are for 'frame life' is related to the material properties.
Here's some stuff to look at: https://en.wikipedia.org/wiki/Ultimate_tensile_strength
- Look down the table at the 4130 steel. A typical frame material. The "YIELD" is the strength where a material goes from elastic to plastic. Going into the plastic region means your frame just bent. So your steel frame yields at 951MPa (a number indicating force x area). If you reach the ULTIMATE number, your frame just broke. Bummer. Notice the number is higher than aluminum. 1110 MPa for steel.
- Now look at aluminum. Yield at 414MPa, Ultimate at 483MPa. You might be thinking - dang, those numbers are lower than steel, AND closer together than steel. I should get a steel bike! Well if you like steel. But what the numbers mean is you need more AREA for your aluminum bike tubes. That's why CANNONDALE made the tubes so fat way back when. And the steel bike tubes were smaller. The bigger tubes are less flexy, so aluminum got the reputation as a "stiff" ride.

- Now! What about carbon fiber? Look down the table some more until you see the graphene / carbon nano tube / glass numbers. I have no clue which of those apply to bikes. But for this post it doesn't matter. All I want you to look at is the yield number: N/A As in not applicable. It doesn't bend - BUT MAN! LOOK AT THOSE ULTIMATE NUMBERS!!
So super light bikes. Super strong. Just a reputation for breaking like glass. No bending.

But wait, there's more! You asked about life. All I talked about was yield and ultimate strength. CLICK HERE: https://en.wikipedia.org/wiki/Fatigue_limit
There a plot showing STRESS vs CYCLES (S -N Curve). It indicates how many cycles of a given stress a material can take. So the big guerrilla sprinting everyday applies really high stress loads. The high stress loads limits the number of cycles you can apply - you have to stay BELOW the S-N curve, or YOU FAIL! bummer.
The guy biking to work everyday and just sits on the bike and pedals, no so high stress loads. He rides his bike for 20 years and never has a stress failure.
Steel is higher than aluminum, but that applies to THE MATERIAL - not necessarily your bike. An aluminum, well designed, will last 30 years - I had a Cannondale touring bike from the 80s. Would have kept it but it was small.

Carbon fiber can take really high loads, so if well designed, well built, I'd ride it as long as it's not damaged. I think the coatings used now days also protect degradation to to sunlight - that could have been another 3-year limit thinking.

Let me know if all that helps. And correct anything I got wrong.... or... everything I got wrong.