>It was a mistake to lower majority from 21.
I don’t think so; 18 year olds are perfectly capable, and IMO culpable for their actions.
I think what was/is a mistake is keeping so many age-groupings:
16 for driving,
17 for enlistment,
18 for tobacco,
21 for alcohol,
etc.
Legally-speaking if something is to be divided/differentiated based on age it should very likely be on majority/minority; the restrictions on ages of political positions s for EXACTLY the reason [immaturity] that it seems you are citing.
You would like an 18-year-old POTUS?
(Not that some would not be better than what we have now,
but what you would probably get is an 18-year-old Obama.)
Oh, also, and an 18-year-old may be capable of feeding and clothing him or herself and procreating, but making rational decisions about government before they have a few years of real life under their belt is another matter.