Advice please?

Sorry to cross post but I have a bit of a dilema.

My cat is now 15 and a half, and although she still eats, drinks, toilets,
and sleeps, she does little else. I know she has some discomfort from her
lower back, and probably kidney problems. Other than that and her increased
laziness she seems quite happy, purrs when I go to see her, still comes down
stairs when she wants more food (I have 3 cats, and they decided they wanted
to divide themselves so Katie got the upstairs, and Gizmo and betsy the
downstairs), she doesn't come and see me in bed any more and has no desire
to go outside (even though she used to go out days at a time).

I realise this is just old age and everyone slows down, but I'm wondering
whether I should consider taking her to be put down in a month or so rather
than forcing her to go on living when basically she doesn't live the most
active of lives, and its not that full of interest. I try and spend time
with her, but I just wonder if forcing her to live on to the bitter end is
the kindest thing to do.

Any thoughts would be gratefully recieved.