That can be easily solved by the coin control features already integrated in the mintcoin wallet. I already use only the recent coins for transactions while my older coins are staking. I think people will understand if their coins are staking so they can't use them instantaneously. Or that they lose their coin age by moving the coins. Most people will just put the money in their wallet and let it there and collect interest. Just like a bank.
I see what you mean now...
I wonder if there's a way to implement something that will basically tell the wallet which transactions were staked (or at least which address it was from). This would help if the bank were to give each person only the coins they themselves were staking. I don't believe this happens right now though, but may be worth looking into if equality is really that important with this.
Thats why i think its a good idea to require them to leave the coins untouched for a couple of days more than minimum coin age. It would give a higher possibillity to stake and one wouldnt need to keep track on whos coins actually staked. Simple and safe