My wife (39) took DHEA for our third child (born this week).
I really, really think it helped. We'd been trying for a year to conceive naturally without success. Our IVF clinic had suggested we not go for more treatment, due to low AMH results.
My wife's low AMH, began rising after only 1 week on the DHEA. After 2 months on DHEA, the ovulation tests were coming back much more strongly, with well-defined lines. Previously, the ovulation tests almost always gave a faint line, whenever they were done.
In the third month, we fell pregnant, and have just had a baby boy! This is our first naturally-conceived child (previous two were IVF).
In the States, they recommend 75mg of DHEA per day, loaded towards morning/midday (as it can affect sleeping). We went for 50mg, half in the morning, half at midday.
Some links below regarding DHEA studies. Some of these studies have been done by scientists who hold patents in DHEA-related treatments, so that knocks their credibility somewhat. Still interesting reading though:
www.ncbi.nlm.nih.gov/pmc/articles/PMC2992540/
www.sciencedaily.com/releases/2010/07/100701145535.htm
www.rbej.com/content/9/1/67
rd.springer.com/article/10.1007/s10815-007-9178-x
www.rbej.com/content/8/1/140
www.rbej.com/content/9/1/116
www.ncbi.nlm.nih.gov/pubmed/20638339
(May need to copy-paste the 4th link in the list, rather than clicking it).
Obviously, none of this is scientific in any way. Just our experience, so please read with the due amount of skepticism! 