I think it makes sense to build something that you want to use personally (solves one of your problems), or at least solve a problem that you understand well. It's certainly possible to do neither but I think you're more likely to fail then.

It has nothing to do with age.

The advice about not trying to compete in the "web2.0/social" market I think is a good one regardless of age. It's far too crowded with "me too" startups and most business models rely only on advertising or hopes of getting bought before loosing too much money.