If age is not a factor
It isn't.
So currently according to
https://twitter.com/CoreFeeHelper, a 11.7sat/byte transaction should get confirmed between 3days-1week. But if miners simply choose those transaction with the higher fee (and assuming than mempool size remains the same), then this transaction should never get confirmed as the higher fee transactions will always get priority over it. So maybe we're missing something here.
What you are missing is that you are making the assumption the mempool size remains the same.
We cannot predict what the mempool will do. We cannot predict how many new transactions are going to be broadcast, how large those transactions are going to be, or what fee those transaction will pay. We also cannot predict whether we will find 10 blocks in the next hour or 0 blocks in the next hour. As such, every fee estimation is exactly that - an estimation.
Having said all that, an estimation of 11.7 sats/vbyte for 3 days is on the low side, since we've got over 90 MvB of unconfirmed transactions above this level already in the mempool, even without taking in to account any new transactions. For 3 days right now I would probably go somewhere around 16 sats/vybte which is about 20 MvB from the tip, but it's still just an educated guess.
The thing to do is to make sure you enable RBF on all your transactions. That way if it gets stuck, you can easily bump the fee higher.