I thought that was an interesting article. Penny has a bit of a dodgy record on certain issues, but this was well worth a read.
I have been with my DH since I was pretty young and while I have no regrets whatsoever and would change nothing, she's right on the money that energy you pour into relationships (and having children in your 20s as I did) is energy that isn't available for other things. Being in a sufficiently serious relationship that you also think of someone else when you take your major life decisions inevitably imposes limitations, just as it also (ideally) provides you with a lot of support you wouldn't have otherwise had.
Having only really been single as an adult at university and for a little while afterwards I can't really look at my life and identify periods of greater or lesser achievement, but it was useful to reflect on the difference being partnered early has made. Not all negative by any means, I don't think Penny does a very good job of covering that part- probably as she hasn't experienced it. But I think the topic is something we could generally benefit from thinking more about.