What is the phrase or idiom for older people who still can function properly

Sentence: Although he is 90 this year, he still _____: he can still walk, eat on his own.

Solution 1:

Sprightly is often used in that sense:

(especially of old people) energetic and in good health:

• He's a sprightly old man of 75.

(Cambridge Dictionary)

Solution 2:

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"

Google Dictionary

After missing out last year with a cold, the Queen’s back in style ... and Philip’s as spry as ever

Daily Mail 25th December 2017

Solution 3:

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:

self-sufficient

Meaning:

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.