I hire IT guys in consulting all the time. Here's my personal take on the interview dress "rules":
Regardless of your role and regardless of the fact that workers there may wear shorts and flipflops, the most casual you should ever wear is dress pants, dress shoes, a button-up shirt (not a polo). And that is OK only when (1) you're invited to not dress up and also (2) you're not in a customer-facing or management role
Otherwise, sport coat, dress pants, etc. -- that is appropriate dress for (1) customer or management facing role who was invited to not dress up, or (2) any other roles -- server room guys, networking dudes, unseen developers in the cube farm
Anything else - suit.
I can't say that someone being especially well dressed ever mattered all that much when hiring, except maybe for a sales position. Ill fitting, oddly dressed, out of style, not ironed, etc. -- well, yes, negative influence can happen.
Regardless of how dressy is the correct choice, hiring manager is probably wanting someone thorough, attentive and careful with detail, so make sure there's some visual indication of those attributes...
Yes, there is such a thing as overdressed -- if you're showing up in Kiton and Edward Greens and the CEO wouldn't ever have done that, wouldn't have any occasion to do so -- the only image that projects is that you didnt try to find out much about the company beforehand.
None of that varies by age of the person.
Hope that helps - Good luck