Here's what we know:
* He's been alive for a very long time: Mr. Popo has been the guardian of Kami's Lookout for centuries, as confirmed by the anime and manga.
* He's powerful and knowledgeable: Mr. Popo possesses immense strength and mystical abilities. His wisdom and knowledge suggest he has lived a very long life.
* He's a close friend of Kami: Kami, the former guardian of Earth, lived for thousands of years. It's likely Mr. Popo is similarly ancient.
While his exact age is a mystery, we can safely assume Mr. Popo is one of the oldest characters in the Dragon Ball universe.