Early on in You Can't Take It With You, Grandpa explains that he hurt his ankle when his granddaughter dared him to slide down the banister.
GRANDPA: The thing I like most about it is the crutches, I've been wanting to walk on them ever since I was a kid.
The crutches then are a sign of Grandpa's indomitable optimism and general playfulness. He is young at heart, which is, he shows, more important than being young of limb. Life can't get him down.
There's a twist there, though (and not just of a leg.) The cast in the film isn't real, but the crutches are; Actor Lionel Barrymore, who played Grandpa, had serious arthritis. Capra wanted him in the part, so they wrote in a broken leg for his character. The crutches can be seen as a sign of Grandpa's optimism and playfulness. But they can also be seen as a cleverly scripted acknowledgement of a real injury.
Grandpa isn't halted or deterred by setbacks, but in real life, spirit isn't necessarily enough to overcome hardships, even if you're a spirited old film star like Lionel Barrymore. Sometimes you need workplace accommodation too.