How is heritabiltity of life span useful if by the time the lifespan becomes known (eg at 80yrs old) the inheritance is not possible anymore (eg menopause)?

It's not useful. Indeed that's likely why we die of old age - there is no selective pressure to remove harmful mutations that don't reduce your ability to pass on your genes so such harmful mutations just accumulate over many generations. You might have a mutation that will cause your heart to rupture at age 150, but you'll never know it because you'll die of something else first.

It is possible though to selectively breed animals like flies for long lifespan. You wait to see how long one generation lives and cull the descendants of those that died early. It's inefficient but lifespan extensions of 50-60% have been demonstrated. One could imagine through gene editing that a species might be able to reap the benefits without the culling.

https://pubmed.ncbi.nlm.nih.gov/3113991/

Heritability acts on lineages, not individuals (in general, not always) - a good rule of thumb is that traits that benefit 3 or more generations of a family have a good shot at being propagated. In this case, the advantage (of both menopause and longevity) is increased well-being of the tribe, ampliyfing the positive effects of culture and stability. Wisdom of the elders is implicit to the genetics. This is a tradeoff with the cost in resources; at some point the cost to keep someone around might exceed the benefit, but from an evolutionary standpoint, the accounting is over a lifetime; in a relatively stable environment, genes that improve longevity and healthspan will be reinforced by the positive feedback loops of culture and nurture and civilization and technology. Menopause is also prevalent in orcas and a handful of other mammals - and older females help rearing and protecting babies, and so forth, with a protoculture providing that feedback loop.

Note that evolution isn't about individuals, it's about genes (which we should further note is more than just DNA, but that's a different discussion). If it weren't valuable for humans to live at all without being fertile, then the average age of menopause and the average age of death would likely be much closer together. As it turns out, the human genes that were best able to pass themselves along were those that kept old people around despite being infertile, presumably to the benefit of helping to raise grandchildren, among other things.

Explanation I've heard in popscience books:

Healthy grandparents that are around to support their children and take care of grandchildren increase the fitness of the entire lineage by helping their children have more children and those grandchildren to be healthier/safer.

We're social animals. Anything that benefits the extended tribal unit is advantageous. Adults beyond child-bearing age contribute significantly to child raising, education, leadership etc of the entire tribe.

Similar effects are seen in other species

https://www.nature.com/articles/s41467-018-05515-8

In (quantitative) genetics literature, heritability is usually defined (simplifying a bit) as the proportion of variance of a trait (lifespan, height, etc), in a population, that can be explained by genetics. The rest, by environmental factors, or error.

If height were a 100% heritability means that all differences in height between individuals would be explainable by genetics.

What is the question you are asking? What does "useful" mean, in other words? How does it contribute to the reproductive success of the offspring?

the more little old ladies around, the easier it is to raise kids.

It's probably not that useful (evolutionarily) beyond some age. Old people consuming resources without adding anything or holding back societies.