One benefit of a stair lift is that even if your legs and lungs still work reasonably well you can use them to carry things which for people who may suffer discomfort from lifting or carrying makes life easier. A stair lift was installed for my father when he became too weak to climb stairs but he probably only used it a couple of times a day, in contrast my mother used it up to a dozen times a day for moving laundry/vacuum cleaner ,medical supplies, kettle and supplies for morning tea and loads of other things. Without it she may have struggled to look after him so easily and exacerbated her own painful hip problems.
She has moved since and twenty years on has installed one for her own benefit, sometimes she feels fit enough to choose not use it but sending things like some library books or her IPad up first means she has two hands free to steady herself should it be necessary which could save a fall with all the attendant problems that can cause in elderly person.
That for me if we didn't live in a Bungalow would be good enough reason to get one sooner to make life easier/safer rather than wait till it gets essential. And being realistic you may get some decent wear out of an expensive investment rather than just a couple of years before you snuff it.
Someone mentioned up thread that a good local independent installers are more desirable and often less expensive than one of the national chains. This is probably true and they may fit a used unit more eagerly than a large firm and will maintain it cheaper, it is the route that my Mother took after having one recommended and having him service the previous lift. He would not fit refit the previous one as he said it was too old, 3 or 4 years rings a bell but don't know if this an Industry good practice recommendation or just his personal way of working. Anyhow he found an 18 month old chair that would suit though the track had to be all new as it had to go around a 180 turn as it rose. For that model a http://www.platinumstairlifts.com/products/curve-stairlift/ the track is made to order and at the time there was a bit of a backlog so mothers installer went up to the factory from Devon to Yorkshire and assembled some of the parts himself to speed things up.
One caveat with an independent though is that they may be a sole trader. Good as he his Mothers installer was on Holiday between Christmas and New Year when the chair misbehaved and needed attention. He tried to talk me through some steps on the phone to resolve the issue but with the limited tool kit that mother has in the house I could not get into the innards beyond checking some micro switches had not shifted. So mother had to wait a week* , a national will have engineers available all the time though the service plans are costly.
* In dire need he probably would have attended but it was a case of "It's my only break I take each year,is there anyway you can manage?"