Creatine after 40
Creatine monohydrate is the form of creatine that has been studied the most. It is used to be able to lift again, and to keep strength as the sessions repeat. It is not a cure for ageing, and it does not undo a sedentary year.
After 40, the reason to lift is simple. Muscle and strength are easier to keep than to rebuild later. Creatine is an addition to those sessions, not a replacement. On this site it sits in the strength protocol: two full-body sessions a week, and roughly 5 grams of creatine monohydrate a day if you want the add-on. The gram count is the dose used in most training studies, not a personal prescription.
Choose monohydrate with nothing blended in. Jars with extra powders, flavouring and a new name are not better creatine. They are a more expensive serving of the same substance, or a smaller one diluted with something else. Read how much monohydrate you actually get per dose.
The effect shows up in your sessions, not in a feeling on day one. Expect a few weeks with the same exercises before you judge whether the sets got heavier or longer. One week without a difference means nothing. A month without heavier lifts means the creatine had nothing to help.
Skip it if a doctor has told you to. Start with the training. Creatine is the add-on, not the plan.