Worried about her age because of Joe's. might not be bad considering Trump is old, in terrible shape and prone to looking on the verge of collapse at times. But, the idea is Biden is the transitional leader, the guy that brings the country together, unites everyone and gets things stabilized until a more dynamic person takes over after his first term. I think Warren would be a great President, but then we're talking about her running when she's 75 against a lot of younger opponents and risk a rough primary (which is what the GOP always wants).
I'd rather see a younger woman. And all things being equal, a POC.