Personally, I would not top-up the CPF SA at aged 21 as you still have over 30+ years to go before you can use the money! I would say to keep your money liquid in the time before you reach the withdrawal age for short term use. Yes, the compounding effect can turn you into a millionaire on paper, but the funds can only be accessed after you're 55. Think about your own goals and needs. What would you be doing in the time before you can withdraw that money? What would you do with that large sum of money at age 55?