Both Biden and Trump require geriatric medical care considerations because both are elderly. That shouldn't be the issue and nor is the occasional memory lapse.
There is nothing wrong with voting for an elderly candidate if their values align with your values and they are capable of understanding and acting rationally (and with caution) on professional advice.
In the US situation it is absolutely essential that elderly candidates nominate a running mate BEFORE the primary process begins. That way voters can chose a team in the knowledge that their selection remains valid if one candidate is unable to continue for whatever reason. This does not happen for non- incumbents .... and so voters don't know what's in their lucky dip parcel.
