In my opinion, in Western societies, it seems like adolescence is the peak of human happiness for it is seen as a period in which one has many positive relationships with little worries.
Of course, however, this belief is highly ideal and unrealistic.
Nevertheless, adolescence is seen as the time in which we are able to form the most human connections (and thus, justifying the phenomenon of 'young love' or 'puppy love'). Adults usually want to be able to live free and fall in love for (as I see it) that's what peak happiness looks like in Western societies: being responsibility-free and in love.
Since it is viewed that these factors are only ultimately and most completely fulfilled during adolescence, it is understandable why older generations would want to prolong this period.
The period of supposed peak happiness.