How old/or with how much in CPF (OA & SA) should one decide on optimising their CPF schemes for retirement?

I dont think it got to do with age or how much in CPF OA or SA. If you decided to optimise, it is best to do as early as possible to acheive the FRS at SA and let the compounding magic works for you.

However having said that, one shoud not be too rigid in thinking when it comes to housing finance. If your salary and expenditures do not allow you to finance the property by cash, pls tap on to the CPF OA. Once your salary increase and expenditures are controlled, you can always start servicing more with cash than CPF OA.

I set my mortgage deduction using CPF OA to $100 per month and pay the rest via cash. In events where I forsee any huge expenditures in upcoming years, I can always re-adjust the mortgage deduction accordingly via CPF self service.

There is also another school of thoughts that cash is more fluid and should be spend only on investments which can beat the 2% or 4% interest and not waste time on building CPF. At the end of the day, you need to think if you are convinced with using CPF as part of your retirement roadmap tool. ​​​

You don't need $20k in OA to earn the extra 1% interest, all you need is combined $60k balance in your OA (cap at $20k) + SA + MA and you will enjoy the full extra 1% interest ($600).

If you don't have enough cash for housing, it is perfectly fine to use OA for downpayment and mortgage repayment. Just set aside sufficient amount for your housing needs and any excess you have the option to transfer to SA (an irreversible process) or invest the excess above $20k.