Recently worked for a (bad) silicon valley YC company, and was shocked to learn that 6-8 years of experience is considered senior these days. Not enough experience at that point to really be senior, but I think there's such a desperate need for developers that they have to do that.
To be clear, I've certainly met people in their 20s that have senior-level ability, but they are by far the exception.
I agree with you that the "senior" title is passed out too easily, but as someone who is 46 this has been the case throughout my entire career, I don't think it is a new problem.
In fact, the 6-8 years of experience you quote is far higher than I would have guessed if you asked me when the average company (incorrectly) starts handing out "senior" titles.
I definitely agree - more precisely, I meant that "6-8 years" is the time by which nearly all people have attained "senior" designation, even if they aren't that good. However, it used to be (in the places I worked, at least) that a person wasn't senior until at least 10 years of experience.
To be clear, I've certainly met people in their 20s that have senior-level ability, but they are by far the exception.