what causes my dahlia flowers to turn brown
The leaves and stems are green, but the flowers are turning brown.
Dahlia flowers don't last a very long time, so old age is a good possibility, at least with the foliage looking good. You can look for thrips inside the flower buds. They're little insects that remind me of fleas, and some of them love to eat flowers. This article has more info; https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-thrips.htm