Finally? They moved Graterol to the pen at age 20, after only 44 appearances in the minors. He debuted in the MLB pen just days after his 21st birthday.
Alcala was a late sign by Houston (age 19), didn't pitch in full-season ball until he was 21, was acquired by the Twins at age 23 and was moved to the pen after 25 appearances in the Twins organization, debuting in the MLB pen shortly after turning 24.
The only meaningfully faster path would be to put these guys in the pen immediately after signing. And to be fair, more and more successful relievers have that background -- the Rays current pen was pretty much entirely drafted/signed as relievers, for example. But they're not pushing everybody in that direction -- they rescued Glasnow from the Pirates bullpen, for example, and in terms of prospect ranking he might be a reasonable comp for Graterol, justifying the starting leash the Twins gave him. Through age 21, you probably could have made a case for Snell moving to relief too.