I think there's a difference between providing everyday life advice and writing an influential philosophical work.
For example, having a family. I have no idea if Sam has children, but many 30 year olds (like myself) do not, so I don't consider myself able to give life advice to anyone who does, because their life and my life are dramatically different.
In general, I think, it's not that 30 year olds can't give life advice. It's that I'd bet 60 year olds can give considerably better life advice. When "the days are long but the decades are short", it's worth considering that the author has only spent one of the latter in the work force.
That probably just means that a lot of advice doesn't apply to everyone at all stages of life. Someone with kids could write a similar list with advice that would be good for some and bad for others.
I bet a lot of parents, in good faith, give their children bad advice. For instance, "get a good education so you have something to fall back on". That may be good advice for some - probably even the majority of people. For others it will be terrible advice.
Knowing the background and credentials of an author (age, experience, works, kids/no kids etc) can be valuable and guide the reader. It may help identify whether the advice is relevant and applicable to him/her, but it shouldn't discredit or disqualify the author or work in itself.
In this case, Sam just chose to share the answer he gave a friend who most likely will find it more valuable than someone who is living an entirely different life. He didn't say that it applies to everyone, and I'm also certain that he knows that he will learn more life lessons. Nothing is final and I'd certainly expect to see different advice when he turns 40.
The fact that we continue to learn throughout life is not a reason for not sharing what we've learned so far, particularly if we believe others can benefit from it.
For example, having a family. I have no idea if Sam has children, but many 30 year olds (like myself) do not, so I don't consider myself able to give life advice to anyone who does, because their life and my life are dramatically different.
In general, I think, it's not that 30 year olds can't give life advice. It's that I'd bet 60 year olds can give considerably better life advice. When "the days are long but the decades are short", it's worth considering that the author has only spent one of the latter in the work force.