Older players persistently "fail to impress in training"

And their attributes drop very fast, even mental ones. This is happening every month even though you're having a private chat about it every month and they apologise every single time. Yet nothing changes.

What do you do? Get rid of such players?

They're getting older and declining, they simply can't sustain the levels younger players do. Have you tried giving them more rest between games? A day or two off training can help a lot with recovery and with prolonging their careers, train too hard for too long and it will take its toll especially as the players age. It's a bit limited by the capacities of the game at the moment that you really can't just stem the tide of decline, it will continue and the lower a players natural fitness the quicker he will decline usually.

have a private chat with them. older players will see their attributes drop but i find that if you have a quiet word and tell them to step their training up a gear they usually go a lot longer before dropping again