No I do understand perfectly, you are German, not American. It is just as inappropriate for me to be butting into who you have selected to lead your country and you have definitely picked some doozies as it is for you to butt into what Americans do. Saying your president doesn't do anything is not relevant because Joe Biden doesn't do anything either.
If you look at the leaders of many countries Trump is not feeble, or dim witted, or incompetent. So trying to make his age an issue just makes you look foolish. If American citizens like him they can vote for him, if not don't. But you don't have any say in it and criticizing the choice the Americans make will only bring an equal amount of derision on you and the choices that Germans make.
Already your country has looked very foolish. Trump warned them what would happen if they got the pipeline from Russia. They laughed at him and now the joke is on Germany.