its tricky too say when the youngest one should trip is....
its easy to say one should hold off til one's brain's done growing (or so they say 18-20ish)
however, i felt in my own development, that my early trips from 16-18 were pivotal in nudging my brain growth in the right direction. not to say i wouldn't have wound up decently intellectual without psychedelics...but i always had this feeling (once i realized the true personal-evolutionary properties of psychedelics) that i needed to 'expand my mind' a lot before i was done growing. and i can't say i'm disappointed in the way my intellectual facilities have developed.
it seemed i could feel 'where i was going' in my development, and it felt as though my drug choices were apart of who i knew i needed to become....(my drug choices being not only psychedelics use, but also absolute abstinence from alcohol, pharmaceuticals and ecstasy)
part 2: when is too old....
i always had a feeling of sometime it might be necessary to 'get off the carnival ride' and let my self solidify once and for all....but that thought was mainly prevalent when i was tripping weekly. as i've toned down my use of psychedelics and become more reverent and ceremonial in my usage of such, i feel that there isn't necessarily a time that i won't ever need to evolve myself again.
i guess it depends on the person, and their personal reasoning for using psychedelics. if one is tripping for recreation, i see it more likely to 'grow too old' for the experiences...not to say that that would happen, but i think that is the usage which is conducive to growing out of it.
for those of us who partake for healing or evolutionary purposes, it seems more likely that the time to 'never go back' to psychedelics isn't as easy to determine as being too old for it...it would come down to one's individual life quest, and the necessary ingredients to make the most of such quest

which in that case, it would have to be 'taken day to day' in terms of deciding if it is right.
i don't see myself ever declaring 'never again'...unless i am told to do so by my higher sources(don't ask don't tell

), it would take the the obvious understanding of the effect of abstinence being more harmonious, in which case i could never go back, to server higher harmony and purpose :D