It seems that in the next election, Biden will again face Trump and this will again be a fairly hot election, considering that these two pairs are people full of controversy. It will be quite difficult to choose who will win, whether Biden or Trump, but considering Biden's declining health, this will be a good chance for Trump to regain his position and remove Biden from his position as president.
I am not sure whether health of either candidates (let's not forget the fact that Trump is 77 years old and this is no longer 2016) will be a deciding factor during the 2024 POTUS elections.
They don't use health or age barrier because if these have been effective, there could have been many of them who wouldn't be qualified for such race base on their age or health status, as regarding the health, let's lay more emphasis on it this way, if any of the candidates have an health impairment, they will not publicly announce such because they believe it will be an hindrance towards thier ambition, they had prefer to go silent about it untill they arrived the position after winining election, we can gamble and stake on any of them in this regards.