you gave a good example of messi, not all players get used to it when they go from one league to another. messi at PSG is not being the same messi and that is not because of his age and precisely because of the style of play of PSG and france, I believe that for example if messi had gone to a spain team like real madrid or atletico madrid he would continue to play well and scoring a lot of goals and you wouldn't notice any change, but in france or even if he had gone to england or even italy he would have had a very average performance, in the case of cristiano ronaldo is the type of player you get used to in any league, but that's because he started in portugal where he has the same style of play with france, england, germany and italy
Many people and players were daring Messi to go to another club to know what he truly is, and you can see what is happening now. I agree with you that it is not about his age, but the lack of adaptability to the new team and environment he finds himself in my view. Messi is still performing better with the Argentine team but why not replicate this with PSG? It is simply because of his trading psychology. Spending almost all his career with Barcelona and Argentina has affected him psychologically unlike C. Ronaldo that has diverse experience.