Look there are a lot of reasons why progressives should criticize Biden, like the whole genocide thing. But him being super old isn’t one of them. Oh he might die in office? Okay! Cool. More conservative democrats should be on the brink of death. I support that position
My worry with an old geezer president is less that they might die and more that they might start to go senile and/or develop dementia and refuse to step down.
Of course the point is moot with Biden because the alternative is Trump who is absolutely beyond horrendous and has major cognitive issues.