Re: Tin Roof (circa 1903) - Life expectancy?
Standing seam terne metal roofs can be maintained indefinitely, if you're willing to keep up the painting schedule.
I live in a region where tin roofs are ubiquitous; my house has three metal porch roofs all dating to about 1906. I lived in a brick house for a few years that had an 1850's metal roof; the edges needed to be replaced because they had weathered away, but it's always possible to add new metal. (it's just not particularly pretty)
But if the roof has had coating or tar applied, then it has in fact reached the end; coatings are evidence that it is pinholed to a large extent, and tar is acidic and hastens the decline.
Remove not the ancient landmark, which your fathers have set.