Materials/manufacture/production methods/quality, bike use and storage conditions are prolly major factors in the expected lifespan of a bike, mtb.
Anecdotely, My '04 Spec Epic FS and '04 Spec Stumpjumper HT, were both bought by me, used (quite hard usage...) in late 2005. I replaced substantial components, wheels, but the frames were undamaged and aside from minor scrapes, had intact frame condition.
I rode them alot for about 4 yrs, rough terrain, smaller air and drops, maintained them regularly. Rode them much less 2010 thru this past summer. No major crashes, no very difficult terrain.
I've gotten back on them this past summer - very frequently. At a strong level - for a 72 yr old... LOL! I'm breaking much sooner and easier than any of these bikes might.
Because I maintained the 'mechanics', and the suspension components were at a better level, They both ride very well considering their design.
I updated both from 26" to being 27.5/26 mullets (along with some other dimensional tweaks), and found them to have a huge improvement. BUT I've also recognized that a HT is just much less fun riding for a 72 yr old. LOL! The old 26' HT will be sold.
If one maintains a bike, replaces the worn out parts, you'll have a bike at least as good as the original design level, for a long time. Prolly better if you successfully 'mullet' a 26.
I do have recent direct ownership and experience with an almost 'new', better level ALU FS 29r and a Upper Level 27.5r carbon FS - the ALu FS 29r will be sold...
The older Spec Epic Mullet will be kept - a fun bike to ride, and quite nice to have as 'backup' I expect to have both FS bikes be placed with me, in my pyramid.

A search of direct science on the specific topic of Aluminum as relates to MTB frame, did give me one reference I found worth reading.
Material and Design Optimization for an Aluminum Bike Frame

Ride On
Yuri