People die at all ages due to natural causes. It's definitely a higher chance the older you get, but the fact is, some people are healthier at 80, than others are at 40. That post was clearly ageist. It disregarded the reasons that someone may like Bernie, and put the reasoning that they shouldn't, purely on age. That is ageism.
The fact is, Bernie can be healthier than all the other candidates on stage. It's unlikely. But why does that give us the moral permission to disregard him, purely based on that? Why can't someone just like Bernie more, and not throw it all away due to age?