The evolutionary reason is a negative one, not a positive one. It is not that old people need big ears (other age groups could profit more greatly from big ears, whereas, if old people still need big ears, they probably are too old to profit.) Rather, big ears don't do oldsters any harm. And in particular they don't harm their reproductive prospects (after all, what reproductive prospects do old people have?) Therefore there is no evolutionary selective cost if the ears go on growing a bit.

If it had been the case that all the descendants of an old person would be wiped out as soon as his or her ears began to grow in old age, then this trait would soon die out, but the penalty would be both a drastic reduction in the population, and the loss of a lot of valuable genes that happened to be accidentally associated with late ear growth. This would be an example of what we call a selective burden or cost.

So, as there would be a selective cost to keeping tiny shell-like ears, and no penalty to letting them grow, there is no adaptive pressure to keeping them small.

Also, by letting them grow, perhaps we are preadapting for the development of free flight or the harvesting of wind power among retired people.