I do not think much would happen - the bacteria change as you age for a reason, for example the cell profiles in your colon change and favor specific bacteria to live there, So even if you tried to change that then there would be colonization resistance and your gut would potentially revert back to its previous state over time.
What would happen if you adjusted the quantities of microbes to levels consistent with someone younger?
Alternatively, what if an older person lived with a group of younger people? It has been observed that microbes related to obesity can 'jump' within members of a population.
I do not think much would happen - the bacteria change as you age for a reason, for example the cell profiles in your colon change and favor specific bacteria to live there, So even if you tried to change that then there would be colonization resistance and your gut would potentially revert back to its previous state over time.
Now, on the other hand, it is important to note that transplanting feces of obese mice to lean mice resulted to them becoming obese, even by continuing eating the same amount of calories and type of food. That is a totally different ball game