Although the Habsburgs basically gained a monopoly on the title of Holy Roman Emperors following the election of Frederick III, they still typically had their heirs designate elected as King of the Romans by the electors, although it wasn't an absolute requirement. There Habsburgs also seemed to have good luck in regards to the succession as fathers were pretty much always succeeded by their adult sons, or young adult sons (barring Maria Theresa), there being no young successors or regencies.
Was there an age that one needed to be in order to be elected King of the Romans? As far as I can see, there is not' the youngest one I can find is Ferdinand IV, who was elected in 1653 at the age of twenty, although he died and never became Emperor, although he was basically being groomed for the position, being made King of Hungary and Bohemia in name at 13 and 14 respectively.
Could a young heir of the Emperor be elected King of the Romans? If his father died before his majority, would he automatically succeed as Emperor, being that he was elected King of the Romans? How would a regency function? In the direct lands it's easy enough for the Empress Dowager to act as Regent, but what about the other prerogatives of the Emperor outside his lands? Would his regent exercise those in his name, or would they be handled by someone else, say the Arch-Chancellor of the Empire? Or would the young Emperor not succeed at all, and simply remain King of the Romans, leaving the Empire in an interregnum vis a vis having an Emperor until he was old enough to rule?
This scenario would also take place in the late 16th century, early 17ths. (So essentially the 1590s and early 1600s).