As far as I'm aware, age is not a factor in serotonin levels as there aren't a significiantly larger number of older people with depression, but I do know as we get older, it is harder to utilize vitamins and it may be that she is not getting her natural serotonin from food as easily.
Coffee and tea are also vitamin inhibitors so if you drink a lot, or less than 30 minutes before a meal, your body cannot utilize all of the vitamins in your food, which could lead to amemia, tiredness, irritability etc.
That's why I always think detox and vitamin therapy are great 'pick me up's'.
