After thinking a little bit, 12 or 13 looks enough, because you would be young enough to have fun and run from responsibilities, but old enough to be free.
You don't know how younger you will be after every sip, so you decide to go step by step.
Drinking 1st sip makes you a bit dizzy, but there isn't much transformation. Your body hair receeded a bit and your muscles are less toned. Your pants are a bit lose. You're 20-21.
2nd sip is much more intense, you lose a lot of height and weight, and your facial hair is almost totally gone. Your body hair is receeded a bit more and you feel your penis shrink dramatically. Your shirt is now hanging on you and your pants fall down with your boxers to reveal a 6" penis and pubes. You're 16.
3rd sip makes you much shorter. You lost almost 1 foot (30 cm) and you're 100 lbs (45 kg) at most. Your body hair receeded into slight wisps, your last muscles are gone and your penis shrunk even more, now only 3". The pubes receeded to slight wisps. You're barely even touched by puberty. You are a 12 year old tween.
Then you feel a huge energy inside you. You feel like you can run 10 marathons right now.
You are still naked, so you take you wallet and go to the clothes store. You buy an orange t-shirt and a pair of shorts. Who needs underwear?
Then you get into a cabinet and wear them.
At cash point, you see a skate and purchase that too.
You hop onto your skate and start skating around.