Q.Norwegian Crimson King Maple Trees
The trees are very healthy with lovely foliage at the tops, but the lower trunk has more sparse new branches. Since they are planted between the street and sidewalk area, I’m wondering how old do they have to be before the lower branches can be removed safely to let people walk under them, as I a a couple that are quite mature, but we will have to do something about the low hanging branches as people walk on the sidewalk. Can you tell me how to handle this problem?
Thank you. I don’t have anything that I can send a picture of them.

Here is more information on pruning your hardwood trees.
https://www.gardeningknowhow.com/ornamental/trees/maple/growing-maple-trees.htm
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/