Earlier quoted context omitted.
I think their point is not that people in their mid 20's are WRITING LIFE LESSONS, but more that PEOPLE IN THEIR MID 20'S are writing life lessons.
Why not? Experience and maturity is not very dependent on age. Getting those depends on doing it, not just waiting on time passing by. I'm much older than the article author, but it would be foolish of me to think that I couldn't learn things from 20 year olds – including important and profound teachings and advice.
(Give or take some extraordinary exceptions of people who accumulate a lot of hard-earned experience very early in their lives; but the general rule applies).