Certified GKH Gardening Expert
It is best to do this when it is just a little younger, but that doesn't mean that it can't be done.
Usually, you can do this at just about any time, but since it has aged some I would recommend doing this when you are starting to see vigorous growth for the year. Usually you will notice a slow-down during some parts of the year, followed by vigorous growth. This is when to prune.
This collection of articles will help with mango care: https://www.gardeningknowhow.com/edible/fruits/mango