mongodb how to get max value from collections

As one of comments:

db.collection.find().sort({age:-1}).limit(1) // for MAX
db.collection.find().sort({age:+1}).limit(1) // for MIN

it's completely usable but i'm not sure about performance

The performance of the suggested answer is fine. According to the MongoDB documentation:

When a $sort immediately precedes a $limit, the optimizer can coalesce the $limit into the $sort. This allows the sort operation to only maintain the top n results as it progresses, where n is the specified limit, and MongoDB only needs to store n items in memory.

Changed in version 4.0.

So in the case of

db.collection.find().sort({age:-1}).limit(1)

we get only the highest element WITHOUT sorting the collection because of the mentioned optimization.