You can get the age-down potions in three ways: (Supernatural & Generations EP)
1) Discovering Potions at the Chemistry Table where you can discover and craft a Young Again potion. (Generations EP)
2) Fountain of Youth Elixir - at the Alchemy Table (Supernatural EP)
3) Use Lifetime Rewards Points and buy the Potion of Youth (I find this a waste of Lifetime Reward Points since all it really takes is Level 10 Logic and a couple days at the Chemistry Table to create a Young Again potion) - but that may be your only option if you don't have the Supernatural or Generations EP.
And for those who have Hidden Springs, there's also the Leon's Fountain of Youth, which if you use the "Wish For Youth" interaction and your wish is granted, it sets your Sim's age back to Day 1 of the current age group that he/she's in right now. (if your sim is an Adult he will suddenly revert back to Day 1 of Adulthood). The Potions and the Elixir are the only way that you can send your sim right back to Day 1 of Young Adult.