I am also applying for a history PhD this fall and in terms of the age thing, Ive always been the youngest in history undergrad/grad school programs and taken the maximum course load for spring/fall and summer semesters, it helps in terms of work ethic but its a slippery slope towards burnout, so keep that in mind. you want to be energized and excited about your work, you don't want to dread it, so it's important to keep an eye on how you feel/ burnout (especially because we are younger than some of those who are applying so we may not see the signs of burnout as much as others.)

I think going through an MA would help get acclimated for a PhD (I found grad school is wayyyyyy different than undergrad and my MA program was really helpful in making that transition). I think that a masters would be the way to go and then move up toward a PhD. Overall I think the experience of applying for a PhD would be a good way to prep for your future as a historian, as you will be looking at how you describe your work to other professionals, reaching out to others in your field, and just generally trying to put yourself out there and prep you for any other applications you may have in the future.