Q.what is classed as ‘young’ when pruning a magnolia?
I have a 10 year old magnolia that has spread too far, too low and wonder if I can prune it by taking a whole bough off?

I take it your question about "young" is in reference to the article referring to pruning young trees. Since a magnolia can live over 100 years, a 10 year old tree is still young, and you should be able to prune it. If you have any doubts about your specific tree, you can have a certified arborist stop in and take a look. Here's an article that will give you a primer on pruning: http://extension.oregonstate.edu/linn/sites/default/files/pruning_pub_handout2012.pdf