For some young people, there might be some discomfort in admitting this as a relevant source of data about how to live life.

I definitely endorse this. It just wasn't a problem for me and I was generalizing from one example when I shouldn't.

Example changed because the piercing example equivocates possible mistakes by 16-year-olds and 25-year-olds in the 95% figure)

You meant "equates" instead of "equivocates"? Even with that change I'm not sure quite what you mean. Maybe not that important.

In terms of how likely a decision is to be regretted, there's an obvious difference between decisions by a 16 year old and decisions by a 25 year old. Learning that 95% of 60-year-olds regret body piercing doesn't tell us about the difference we care about (decisions by the 25-year-old) because the majority of piercing decisions are made by those (teenagers) we expect would regret just about any major decision. The argument is weaker because the statistic doesn't show what you assert it shows.