That's just a stupid overgeneralization. If you had a 30 year old Curt schilling or Randy Johnson, or Andy Pettitte, or Roger Clemens for that matter, you absolutely extend them 5 years. Jon Lester isn't quite in that group, but he's got a durability to match any of them, and that's the big thing when it comes to contracts.
If your guy has a long track record of durability and consistency, go for it. It might still blow up in your face, but the chance of success, and the upside in the case of success, is worth the possibility of catastrophic failure, which can occur at any point in a pitcher's career anyhow and doesn't magically increase just because a player hits an arbitrary age number
people pay too much attention to raw age in statistical analysis IMHO. It doesn't mean nothing, but if there's extenuating circumstances, then you need to stop behaving as if there aren't. Daniel nava is a classic example, no one wanted to give him any credit because he debuted in his late 20's, but he debuted in his late 20's because he started his path through the minors at age 23, not because he's a scrub like most late 20's debuts.
Similarly with Lester, you have a man who yes, is over 30, but also has never pitched less than 190 innings in a season, and the only time he ever missed significant time was with non-hodgkins lymphoma over 2006 and 2007. that's as safe a risk for a post-30 contract as you're going to find.