Go for the older. You don't have to get a one-hoof-in-the-grave horse by any means. We have an 8 year old horse who is a been there, done that, horse. Super calm, super chill.
We also have a 19 year old horse who is way more high spirited than the 8 year old.
8 year old horses are young, still have many many years ahead of them, but can still be dead broke and bomb proof. I suggest you get with a trainer with whatever horse you get and increase your saddle time from 50 to like 5,000.
My mom wouldn't let me have a horse (even though she had like 6) until I had taken 4 years of lessons. These lessons were 2 hours twice a week. So 4 hours a week and breaks or days off were not a thing lol
A trainer and lessons is a good idea for everyone.