gavra, There are many sources (many but not all listed earlier in this thread) that insist one must become drunk on Purim. There are other sources that indicate that it is permissible to become less than completely drunk, yet even they basically hold that either it is preferable to become completely drunk or at least it is permissible.
Rav Shmuel was talking to Project Yes teens at risk not to the general rabim. This is easily observed if you hang around the shikur Yeshiva bohcorim in Philly on Purim.
Age 21 is the minimum to purchase alcohol, not drink it. Someone brought a “mekor” on another thread a few days ago that legally a child can drink as much as his parents permission allow.
One can follow the shitta of Rovo as described in the Gemora (Megillah 7b) and the Shulchan Aruch (OC 695:2) and Remo, Bach, Tur, the Shelo Hakodish, the Chacham Tzvi, Rambam, Seder Hayom, and the Sfas Emes while still properly davening Maariv and saying Krias Shma with Kavanna on Purim.
And like you said about the Gedolim spouting Chiddushim when drunk – that amazing feat can still be observed amongst Yidden on Purim, from both Gedolim and the average Yankel, ad hayom hazeh on Purim!
Ah Freilichen Purim!