Hello!

Mulch rings are always encouraged without exception on age. Depending on the landscape, if there are a lot of trees, mulching would make it difficult. However, those situations like with pine trees, create their own mulch.

Mulching helps with:
weed control
moisture retention
soil cooling

Common mistakes:
Piling mulch too high (mulch volcanoes). This can create many problems with the trees, especially if the mulch is piled up against the trunk. Mulch should be 3-4 inches deep, but tapered toward the trunk so that no  mulch is touching the trunk. Mulch piled against create a moist environment, encouraging disease and pests. 

Creating too small a diameter of mulch ring around the tree. With a small tree, it should start out with at least a 6 foot diameter so the tree can grow with it. If you start too small, then creating a larger bed later will affect roots. Creating larger beds are necessary sometimes, though, when the tree is significantly larger. If they have surface roots that may be a tripping hazard, enlarging the mulch bed can help avoid that. The goal isn't necessarily to cover the roots, but create a bed outside of walking or mowing. 

Resources:
https://aggie-horticulture.tamu.edu/wp-content/uploads/sites/10/2013/09/EHT-073.pdf
https://www.treesaregood.org/Portals/0/TreesAreGood_Proper%20Mulch%20Tech_0321.pdf
https://tfsweb.tamu.edu/uploadedFiles/TFS_Main/Urban_and_Community_Forestry/About_Urban_and_Community_Forestry/Urban_Forest_Information_Sheets/How%20do%20I%20care%20for%20my%20Tree%20-%20Mulching%20your%20%20tree.pdf