My understanding, not 100% reliable, is that enjou kosai literally means something like "compensated dating" and is indeed used to refer to what we call sugar dating in English and is applied to situations involving adults only. For example, Universe Club staff refer to their service as an enjou kosai one. And enjou kosai clubs are basically dating clubs along the lines of UC.
However, a number of years back (more than a decade I guess), there was a lot of media attention and public scandal involving enjou kosai by underage freelance Japanese girls seeing salary men and such for both money and presents such as expensive fashion-name clothes and accessories etc. Some of them were alleged to be as young as 12 or 13 years old! Whether there was actually a spike in such behavior among early adolescent girls (as the media claimed) or it was just a spike in public attention to the issue, I don't know. Anyway, as a result the term "enjou kosai" came to have the connotation, but not literal meaning, of immoral and illegal prostitution by very young girls.
I think the above is correct, but it would be worth checking with someone who is fluent in Japanese.
-Ww