Many hormones, enzymes, antioxidants and proteins fall precipitously with aging. DHEA is no exception and there is little doubt among researchers that this only serves to acclerate the typical decline that we experience in our senior years. DHEA levels are a tremendous example of this. Check out this (eyeballed) table for just how dramatically DHEA levels fall with age, and that these are not DHEA-S value but rather plasma DHEA: 
|AGE (Years)||DHEA, Plasma (ug/l)|
The key takeaway here is that DHEA levels fall around 85 to even 90% from ages 30 to age 80! That’s a monstrous drop. And, considering that DHEA seems to play a role in nitric oxide production, insulin control and immunological function, it would seem hard to argue that DHEA levels that are about a tenth of one’s youthful peak are a healthy and desireable part of our existence.
Many alternative physicians and naturopaths now consider DHEA supplementation to be a standard part of their treatments. Still others believe that low levels of DHEA should be part of a DHEA Replacement Therapy Program that restores DHEA-S to some percentage of youthful values. Rather than just arbitrarily giving a certain dosage of supplemental DHEA, these practitioners give the amount of DHEA necessary to bring the patient within a certain range of DHEA. This minimizes side effects, which are quite low with DHEA anyway, and insures that levels are physiological, i.e. stay within normal lab ranges.
By the way, Dr. Saya, when he put me on DHEA, said that he preferred the oral form, because it increased DHEA-S, an important reserve “pool” for this hormone. This was echoed and explained further in this Life Extension Foundation artcile: “Upon oral administration, DHEA is mostly converted to DHEA-S, which circulates in the blood far longer than DHEA. Circulating DHEA-S acts as a reserve upon which tissues can draw. Once taken up by tissues, DHEA-S is converted back to DHEA, which can then be locally converted to androgens and estrogens or exert direct action (Samaras 2013; Traish 2011).” 
So what levels do we actually target? LEF (Life Extension Foundation) is probably representative of most practitioners who actually care about DHEA at all: they try to keep men in a range of about 350 to 500 ug/dl, something I discuss in my page on DHEA Dosing.
1) Journal of Endocrinology, 1999,163:1 5, “DHEA deficiency syndrome: a new term for old age?”