I have a old jade plant in a west facing window. Gets lots of sunlight. I water when dry , about once every two weeks. Just enough to moisten the soil. The leaves on the top are turning brown. The plant used to be thick with leaves and branches.
Jade plants are long lived house plants that are very easy to grow and require minimum care. Too much or too little water are the main problems affecting jade plants. The Clemson Cooperative Extension has a helpful article (link below) that discusses how to care for your jade. They recommend that soil should be kept moist but not wet while plants are actively growing in spring and summer. During the winter, let the soil dry between waterings.
Sounds like your jade has done very well for some time with your current watering practice. Why is it now failing? Possibly the age and increased size of the plant have increased the moisture requirements.
I suggest you increase your watering during the spring and summer to keep the soil moist as a first step. If you are not seeing an improvement, the Clemson article provides other considerations to ensure your jade remains in good health.