7 years seems like an entire subjective amount of time to put on a certain material's lifespan in relation to bikes. Over what kind of riding, how often, what terrain, what climate? Just seems artificial to me to try and assign an actual number, why not just say "aluminum doesn't last as long as steel (but also doesn't rust)". A steel frame could easily die in 7 years time too if someone doesn't guard against rust and leaves it in the rain too much, or it could be on the road 50 years later with proper preservation.