[bruce511]

Not to state the obvious, but old people are really, well, old (and frail).

Your pill would need to do a lot of heavy lifting. I'm no doctor, but your pill would need to deal with mental issues (dementia of all kinds etc) and physical issues, like muscle mass, bone density, flexibility, skin elasticity and lots more.

There are very few, dare I say no, 90 year old that could pass for 70 - what sort of condition would 120 bring? Or 130 etc?

As an expert in one field, used to customers waving their hands telling me "this won't take long for you to do, it's very simple", I feel like your belief in (medical) technology, and what is possible, may be just a touch optimistic.

[musicale]

Presumably some sort of cellular rejuvenation - perhaps a way of gradually replacing senescent cells with healthy cells and/or of activating telomerase without causing cancer - would have beneficial effects across your entire body.

There are already large mammals - bowhead whales - that can live 200 years. Perhaps we can adopt some of their longevity traits without having to become giant ocean creatures.

[inglor_cz]

It would likely not be a pill, but a therapy replenishing stem cells, the immune system etc.

The frailty comes from somewhere, like depletion of certain stem cells that produce new tissue.