Could the demographics/average age of homes in an area (e.g. built in the '90s, never updated) affect sales down the road?

We wouldn't lose sleep over it. Older homes sell all the time, and a '90s-built neighborhood is on the newer end of the country's housing stock. Look at where people actually live. A huge share of the housing in nearly every established metro went up decades ago, and those homes trade constantly. Our own homes were built in 1975 and 1973, and neither is hard to sell because of its vintage. Condition and updating matter far more than the year on the permit. Some buyers do prefer newer construction, and a dated home that's never been touched will show weaker against an updated one nearby. That's a fixable presentation issue, though. Kitchens, baths, flooring, and general upkeep move the needle much more than age. So instead of worrying whether a '90s neighborhood will struggle in ten or twenty years, watch whether homes there are being maintained and updated over time. A well-kept older neighborhood in a location people want holds demand just fine.