I rode oversize bikes most all my young life and generally sat more upright. It wasn't till I turned 50 and started riding a lot that I even thought about getting aero. As my speed and distance increased I saw the benefits to being aero. I thoroughly enjoy my Tarmac in the smaller size frame than I've ever had and the low aero position it gives me. When riding at over 20mph, it's very apparent the aero benefit I get in the drops even as opposed to the slightly less aero position I have when on the hoods.
That big watt savings will benefit those that want to do long rides at more than just a pace you'd do for bike touring with loaded paniers on front or back. So I'm never certain why older people tend to want to sit up. At 65 y.o. I'm lower and more aero on the bike than ever.