yes the MC himself isn't fully human,Being a strong mage doesn't mean he can change the nature off someone else's lifespan (at least lengthening it,cutting it short would be easy

)
i won't deny its possible in some way to make some magical item that extends a persons life.
but that would probably be expensive and come with other downsides such as limited effectiveness extending be less effective over time as it would require more magical energy the older said person gets... at some point even a god would have to struggle.