if you go back about 100 years in the US you would find 14-year-olds who are working, married and starting families. I don't see any better age to start a brand new character. In societies where it's common for children to work the farm or go hunting with their parents, seeing a young person with adult responsibilities should not be uncommon. King Louis XIV was considered to have reached his Age of Maturity at age 13 where he no longer needed a regent.
I've started three of my campaigns with the characters in the year of their 'adult naming' ceremony (like a Bar Mitzvah in my world) that happens between age 12 and 15 depending on their ability to find an apprenticeship, acquire land, or decide to leave home to find opportunities in the larger world.
They will still run into grumpy old folks who complain about 'durn whippersnappers' like people my age like to complain about Millenials.
I'm more confused by the players who want to play adult or old characters. Some players also have a long storied past and are still somehow a level 1 character who is about to suddenly gain 10 or more levels over the next few weeks/months.
For my players who prefer to play older characters, I work with them to determine why they are just starting to realize their potential.