Of course 67! - detailed analysis

- Start to take at 67.

- Assuming $72000 balance by 70

If you put the $72000 in a broader market index fund, assuming only 8% annual return,  you can withdraw $599 per month until 90 (by then the initial $72000 drops to 0). That's exactly the $600 the government promises for delaying to 70. (but the government doesn't promise how long one would live - in exceedlingly majority of cases, it's a better deal for the government if you delay to 70)

If the index returns 10% instead of 8%, you can take out $1764 per month for 20 years (till age of 90) - much more than $600!!

(8% isn't unthinkable.  S&P index returns >9% last 30 years).

67 is a much better deal, because you have the money in full control regardless how long you live.