Insomnia, fatigue, mental fog, throbbing headaches. Age related vs stress related, or other causes?

Yes! All of the above. Could also be thyroid dysfunction, depression, sleep apnea, and a host of other things. Typically, stress, lack of excercise, poor eating habits.
Both. Age does cause changes in sleep with progressively less deep sleep as you age. Poor sleep can cause the other symptoms you mentioned. Alternatively, stress can interfere with sleep. At age 39 I would suspect stress vs age. Fix the stress, fix the sleep, cure the other maladies!!