A recent report by the Centers for Disease Control and Prevention on the use of these medications by persons 12 and over from 2005 to 2008 provides some answers. Canadian use patterns tend to be broadly similar to the US, so we can assume that the figures are not too different for people above the 49th parallel.
The full report can be found here.
Overall, the figures are quite interesting:
Percentage of US citizens 12 and over taking antidepressant medication: 10.8
Women: 15.4 (about 2.5 times the rate of males)
Age group with highest prescription rate: 40-59, 15.9%
Men: 60 years and over, 9.4%
Women: 40-59, 22.8%
Teens age 12-17 taking antidepressants: 3.7%
Rank of antidepressants of all drug classes in terms of frequency of use: 3
For persons 18-44: 1
Change in rate of antidepressant use between a 1988-94 sample and the 2005-08 sample: +400%
Relationship between income and likelihood of taking antidepressants: Small but positive (more income, more medication use).
This may mean that higher-income people receive more medical care or are more likely to be able to afford antidepressants. The relationship of income level to frequency of depressive symptoms is not reported.
Percentage of persons 12 and over with no depressive symptoms taking antidepressant medication: 7.6%
Of those with severe symptoms: 33.9%
Some of those taking medication without symptoms may be prescribed the medication for another disorder (such as an anxiety problem); and some may be symptom-free because the medication is working well.
Of those taking a single antidepressant, number who have seen a mental health professional in the past year: 29.3%
Of those taking more than one antidepressant: 48.2%
In Canada, the majority of people treated for depression see their GP only; this seems to parallel the US.