Though there is one solution I can think of. Which is to require a transaction fee on all coins sent that are not a day old. So if you try to send coins that are not a day old (i.e. don't have 576 confirmations), you need to pay 0.1 ltc fee. What do people think about that?
That would probably be ok, however, you can't choose which coins to use so if you had a bunch of older coins and one new one which was exactly the size you want, I think the client would choose that one. We'd need to combine it with a tweak to the coin selection algorithm.
If he's using custom code to create the transactions, he can use whichever coins he wants to.