If Bernie runs and Beto doesn't, I'm all in with Bernie. Bernie "had" the best numbers with millenials in 2016 and given the lackluster quality of the Democratic field I think he will shoot to the top. I don't think he's too old. Bernie's still got the fire in his belly and he's not liable to retire any time soon anyway.
Beto could take those millenials away from him though. Along with people of color and a few more demographic slices of the pie.
I'm not saying Beto would be a better president. Just that he will win in a long, hard fought primary race.
"Be yourself; everyone else is already taken."— Oscar Wilde