He is now. 10 years from now? Who knows. Reagan wasn’t displaying any obvious signs of age-related ailments when he started the job either.
But the question isn’t really “would we be better off with Bernie than Donald?” That’s a no-brainer, of course we would. The question before us is “all things considered, is Bernie the best candidate to lead the country from 2021-2029?” It would be irresponsible not to include his age as one of the factors to consider when answering that question.