With healthy joints, the elderly can move as agilely as the young.

The storyline goes like this. An old but naughty/playful man who is as agile as young people accidentally runs into trouble.

So the old man pretends to be old and staggering: He pretends to be suffering from joint problems, which is quite common among those in his age group. This way he could get out of the trouble easily since nobody would imagine that an old man would be the trouble-maker.