Hi,
According to me, if you made the changes in Useful life of Asset Class using T-Code OAYZ, then this configuration is useful only at the time of creation of New Assets. No change will take place in the existing assets. Even though you have maintained 5 years in OAYZ but still at the time of creation of asset you can make changes in the useful life as OAYZ is at Global Level whereas Asset Master creation is at Local Level. So Local will supersede Global.
Regards,
Malhar.