Wrinkled Onions

Listen up, numbskull. This "joke" is playing on the words "old" and "wrinkled" to create a mental image of an old man's skin. The punchline twists this by saying the old man's favorite food is "wrinkled onions," which is just a gross and kinda morbid association. It's funny 'cause you're dumb and probably think old people taste like onions, which you should know they don't, since you have no friends and probably never even met an old person.