Miners used to include zero-fee transactions in blocks when there was space available. Nowadays, most blocks are full or nearly so, so miners have a financial interest in mining transactions with the highest fee-per-byte first and ignoring anything which has a low or zero fee attached.
There was never any special handling for "infrequent" transfers.
https://en.bitcoin.it/wiki/Miner_fees#Priority_transactions
Maybe I got it wrong: I thought it meant how long the coins have been sitting around, but maybe it means how long the transaction has been sitting unmined to make sure no transaction is left behind?
But it seems I'm on the right track, in that input_age was a measure of how many confirmations, which suggests it's about how long it's been since the last time they were moved:
note1 here: https://bitcoin.stackexchange.com/questions/95407/what-were-...
Only charged money for big transfers and the high frequency traders.
Dunno why it stopped. Seemed sensible to reward hodlers and incentivize use for microtransactions. Ultimately, liquidity comes from being able to trade a big thing easily for lots of small things.