I don’t think you’d mistake a '56 172 with a '25 172S even at casual glance; until 1960 they had a more upright vertical stabilizer and until 1963 they had the “fastback” tail cone rather than the lower tail cone with “Omni-Vision” rear window. At that point you arrive pretty much at the anatomically modern Skyhawk airframe. We fully arrive at modernity in 1977 when they switch to the 4-cylinder Lycoming and pre-selectable flaps.

Then they stopped making them for ten years, and in 1996 they resumed production with the R model and I’m unaware of any significant changes to the hull since. They’ve offered the Lycoming IO-360 in two different de-ratings, they briefly offered a turbo-diesel though I don’t know how many of those actually made it to customers, and they went from steam gauges to Garmin glass panels, the latter of which is available for retrofit, so to tell you the age of any Skyhawk made during my lifetime I’d have to look at the hull number.