I hear the "25" figure used a lot and have even gotten into debates about it in the past.
In general, it's about life experience and not so much about chronological age.
As a rule of thumb, I consider someone who is out of college, working in their adult vocation, financially independent and on their own, they have graduated from being VY[M/W]. This happens at different ages for different people. There are people who are company CEO's at 23 and others who are still flipping burgers at McDonald's and living with mom and dad at 30. Emotional maturity and experience in relationships is another thing that can vary vastly from person to person at different ages.
It really all depends on the person.
Like Abraham Lincoln once said, "You can't believe everything you read on the Internet."