Some are but some are not, it depends on the companion. Clara Oswald seems to age normally if not slower than usual. Danny and Courtney also look quite young for their ages.
And for the original companions, Ian and Barbara are rumored to have never aged since the sixties, and Susan is a Time Lady\Gallifreyan and thus also ages slowly.
Turlough (a Trionite) was also apparently somewhat older than he looked, and Romana was also a Time Lady. They propably also age slower than normal humans.
Adric (an Alzarian) propably aged faster than humans, though. Or would have if he was still alive.
And if any of the human companions is aging faster than normal... quite a few of them were played by actors a bit older than the characters. It's called Dawson Casting. Did Freema Agyeman really look 20 or Arthur Darvill really look 22, or Sophie Aldred really look teenaged? She looked older than Bonnie Langford at times, and Mel was suppose to be 22 like Bonnie was.