Sentence: Although he is 90 this year, he still _____: he can still walk, eat on his own.
Sprightly is often used in that sense:
(especially of old people) energetic and in good health:
- He's a sprightly old man of 75.
My next door neighbour epitomises the vigour of the elderly who are still spry.
He is 94, is up every morning at 07:00, smartly dressed and off to the shops on his mobility scooter. He has just married my other neighbour this year, who is sixty-six.
Joe is one year older than his birth certificate shows since his birth was registered in 1924, a year after he was born in 1923.
Joe is spry.
especially of an old person) active; lively. "he continued to look spry and active well into his eighties"
After missing out last year with a cold, the Queen’s back in style ... and Philip’s as spry as ever
You have two different meanings here.
The question asked in your question title, mentions simply functioning properly, but the example sentence you have posted adds an extra dimension "on his own".
The best phrase to fit the meaning of your example sentence I would say is:
Able to provide everything you need, especially food, for yourself without the help of other people. -- Cambridge.
So your example sentence could be changed by adding "is self-sufficient":
Although he is 90 this year, he still is self-sufficient: he can still walk, eat on his own.
I would however remove the last comma and replace with and, re-order slightly and also remove the second still.
Although he is 90 this year, he is still self-sufficient: he can walk and eat on his own.
Hale is an old word meaning healthy, now largely fossilised in the phrase hale and hearty.
Collins defines hale as “healthy and robust”, and gives the following usage note:
If you describe people, especially people who are old, as hale, you mean that they are healthy.
She is remarkable and I’d like to see her remain hale and hearty for years yet.
I think this has the meaning you’re looking for, and also tends to be associated with age, so it has the right connotations.
The single word hale is usable on its own, but the phrase hale and hearty is probably more common. Either should work for your purposes.
Although he is 90 this year, he still has all his faculties: he can still walk, eat on his own.
noun, plural fac·ul·ties.
an ability, natural or acquired, for a particular kind of action:a faculty for making friends easily.
one of the powers of the mind, as memory, reason, or speech:Though very sick, he is in full possession of all his faculties.
an inherent capability of the body: the faculties of sight and hearing.
exceptional ability or aptitude: a president with a faculty for management.
Some great answers here, but they all take a positive point of view of "functioning properly" - their words indicate function above and beyond the minimum.
Another options would be compos mentis, meaning "of sound mind; sane".
It is certainly used in British English to describe older folks who are still lucid and "functioning properly" from a mental perspective (even if they are otherwise infirm).
In addition to some other great suggestions here, I'd like to toss in "sharp" or "sharp and able" as a contender. I use it frequently to describe my wife's grandfather, who is in his 90s but still doesn't miss a beat and can do most things for himself.
Edit: My own answer revealed another possible answer, "doesn't miss a beat". It refers more to being mentally fit, but also implies being physically able as well.
There's a popular idiom "fit as a fiddle".
Although he is 90 this year, he still is fit as a fiddle: he can still walk, eat on his own.
fit as a fiddle BRITISH, AMERICAN or fit as a flea BRITISH
If someone is as fit as a fiddle or as fit as a flea, they are very fit and healthy.
Note: In the first two idioms here, 'fit' means healthy and full of energy.
He was nearly 80 and as fit as a fiddle.
Note: This expression may originally have applied to a violin player, or fiddler, rather than to a violin, or fiddle. The fiddler had to be fit in order to play all evening at a festival or party.
Alternatively, 'fit' could mean 'suitable' rather than 'healthy', so the original meaning may have been `as suitable for its purpose as a fiddle is for making music'.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012