Social media account activity materially affects the prospects of many people who aren't politicians, especially if you engage in any controversial discussion. It's common for employers to name-search you and peruse your social media history, especially in careers that involve children like teaching, where naive new teachers are often terminated for putting child-inappropriate content on their public profiles, or getting linked to something they thought they'd shared anonymously.

"I was young and stupid" only works if the content in question is trivial or unimportant. If you happen to be on the opposite side of a political debate long since settled, you may end up getting yourself pwned pretty hard, even if you're not a politician.

Sure. However, then either watch what you say when using your real name or use a pseudonym. I don't always succeed, but I try to only say things online that I would say to the person IRL. That makes me think twice about it. I like to think that if someone was to look back through my comments or sites like HN where I use my real name, they would think that I am a reasonably reasonable human being.