Your body and mind are dynamic entities. This means that they need to be used to enhance and maintain them. If you stop using any part of them then atrophy will set in. This means that they will not be as strong next time you go to use them.
If you stop learning, questioning and being curious your mind will begin to suffer. It will become more forgetful, less insightful and you won’t be as sharp as you once were. Stay ahead of this by making sure you keep learning (lifelong learning has been shown to have beneficial effects upon mental and physical health) and also testing your mind (puzzles, games and exercises are great for this). Have regular deep conversations with others and share your ideas while listening to others.
If you stop using parts of your body they will get weaker. Stop walking and your legs will get weaker. Stop doing aerobic exercise and your cardio pulmonary and respiratory systems will be weaker. The longer you neglect these the harder it will be to get it back. It’s not impossible to get physical fitness back but its a lot easier to keep it in the first place.
Don’t think this this is simply a problem for older people. It affects anyone regardless of age. It doesn’t matter if you are 20 or 80, if you don’t use it you will lose it.