Can you please explain to me how merkle trees work?

I know that Merkle Trees are commonly used in blockchain to allow us to efficiently verify the existence of some information. Each leaf node contains the data, and a hash of the data. I don't quite understand how exactly the verifier of a blockchain can tell that some data exists. Like if a verifier is trying to figure out how old you are based on credentials you provide in a document on the blockchain, (is that possible?) how is that done through Merkle trees? Or is it just used to prove solely the existence of information on a blockchain? It's hard for me to comprehend.