Why people age out of online blogging

Two very important related factors:
The valuation of privacy, and the devaluation of history.

Example 1:

You're an adult with a career now. You mind your online presence much more closely. Also, it's much much harder to be anonymous on the internet, now that search engines can correlate almost everything you do and show those correlations to other people. Why risk it?

Example 2:

You've gone through a relationship transition. All the memories, even the positive ones, revolving around your old spouse and their family and your friends, are now a potential source of jealousy, negativity, or embarrassment.

Either way, the solution is to wipe old things out of public view, and thoroughly sanitize anything you place in public later.