Regarding the "...at least until some age" part,
I don't understand what the age of the blockchain has to do with this, probably because I didn't understand the exact method of this "attack" (I don't like to call it attack though since it's not damaging the integrity of the blockchain nor steals anyone's funds).
If I would make a guess, is it because the more people use GRIN and the more transactions are being broadcast, it becomes less possible to trace an individual transaction?
By "age" they could have meant that linking is possible only early in the transaction broadcast process. Once the block is confirmed all inputs and outputs are in one big coinjoin, so it's much harder to figure out what goes where, especially since the amounts are not visible. And after the outputs are spent a few more times it becomes even harder.
And yes, the more transactions in a block the better it is for privacy.