If you had to pick the absolute hardest, most challenging goal possible in biomedical science, I think it might be to alter the genes of adult humans so as to safely extend healthy life. Yet this is pretty much the course of the mainstream aging research community - and so I believe they are setting themselves up for maximal expense and minimal progress:
It is their belief that this is the only practical way ahead: a laborious slog towards complete understanding of aging and metabolism, followed by an even more complex navigation through re-engineering that metabolism to age more slowly. The sheer scale and difficulty of that task is why many scientists feel that meaningful engineered longevity - more healthy years through science - is a long way away indeed.
Here's a paper restating that point:
Studies performed on various experimental model systems indicate that genetic interventions can increase longevity, even if in a highly protected laboratory condition. Generally, such interventions required partial or complete switching off of the gene and inhibiting the activity of its gene products, which normally have other well-defined roles in metabolic processes. Overexpression of some genes, such as stress response and antioxidant genes, in some model systems also extends their longevity.
Such genetic interventions may not be easily applicable to humans without knowing their effects on human growth, development, maturation, reproduction and other characteristics. Studies on the association of single nucleotide polymorphisms and multiple polymorphisms (haplotype) in genes with human longevity have identified several genes whose frequencies increase or decrease with age.
Whether genetic redesigning can be achieved in the wake of numerous and complex epigenetic factors that effectively determine the life course and the life span of an individual still appears to be a 'mission impossible'.
Not impossible, just far, far harder than the alternative - which is to avoid changing human genes and metabolism, rather aiming to repair the damage of aging in the biochemistry we have today, thus reversing the effects of aging. That goal allows us to skip over a great many things we don't understand about human biochemistry and avoid many challenging endeavors - and it will produce more valuable and effective therapies into the bargain.
Why take the hard path to extend longevity a little by slowing aging when there is an easier and more direct path towards reversing aging? The debate over the approach to aging research in the next few decades is vitally important to progress in engineered longevity: the presently dominant strategy is an inefficient path forward, and it will eventually produce therapies that do little to help those of us who have grown old waiting for them. That has to change.