The label on 5-HTP bottles often state that it's not suggested for use under 18 years old, but that's just your typical legal liability warning. Realistically, 5-HTP shouldn't be used by anyone under 40 or 50 years old. The compound is only slightly effective as a preload to an Ecstasy trip, unless the user is already "understocked" from a previous roll. If the user is relatively healthy, the excess serotonin will have nowhere to go and the 5-HTP will simply be removed from the body in usual fashion or the serotonin reused by the body for something else.
Typically 5-HTP should only be used to counter dimished serotonin production which accompany other middle-age ailments, and to replenish serotonin stores immediately after a rapid depletion, such as in a roll. For frequent MDMA users, prime concern should be focusing on Vitamin C and water replenishment. Serotonin will restore itself to a good equilibrium over a period of time with no harmful interim effects besides a little emotional slump and mild depression, but the short term effects from Vitamin C deficiency or dehydration can cause serious injury if overlooked.