Edit: I think I understand the problem now. PG's actual advice has always been "finish college before starting a startup". Somehow somewhere that got distorted into "don't start a company in your 20s", which is certainly not what PG has been saying (the average graduation age is somewhere around 23). https://news.ycombinator.com/item?id=45372572 quotes a representative passage, and if you read that essay you'll see that he's mostly saying "don't start a startup at 20".
(Edit: I deleted my last sentence which was irritable.)