What is the expected life span of a mugo pine

Question

Planted small group of mugo pines 10-12 (maybe 15) years ago. Did fine, but now they are starting to die. Old age?

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Answer 1

These trees are expected to live up to 50 years, so I don't think it is old age that is killing them.

That looks like it is winter burn. Are all the trees browning in the same pattern? Has your area had harder than usual winters or has something around the trees changed that might change the flow of wind. This article will explain more:
https://www.gardeningknowhow.com/plant-problems/environmental/winter-burn-in-evergreens.htm

It may also be a fungus. If the trees are browning in different patterns from each other, haphazardly, this would be an indicator that the problem is fungal. If this the case, a fungicide should help clear it up.