What is the antonym of whippersnapper?

There isn't a single perfect antonym for "whippersnapper" because it's a rather informal and somewhat derogatory term. However, depending on the context, some suitable alternatives could be:

Formal and Respectful:

* Elder

* Senior citizen

* Veteran

* Mature individual

* Experienced person

* Wise person

Informal and Positive:

* Old timer (can be affectionate)

* Old hand (implies experience)

* Grandfather/Grandmother (if age is the focus)

* Wiseacre (can be playful)

The best antonym will depend on the specific situation and the intended tone.