You use highly accurate techniques to learn the time when lots and lots of different volcanic rocks solidified from melted rock.
You do this for many different rocks across the continents, and many different rocks across the sea floor. You will find that (note that "older" rocks are those that solidified more years ago, and "younger" rocks are those that solidified fewer years ago.):
The sea-floor rocks are typically younger than the continental rocks, because sea-floor rocks are taken back into the mantle at subduction zones about as rapidly as new sea-floor rocks are produced, while continental rocks are not taken back into the mantle at subduction zones.
The Groins are hard structures and used to maintain uplift beaches as to restrict longshore sediment transportation form tasking place. Are meant to capture sand transportation by the currents.
While the jetty is another type of shore perpendicular structure that is placed adjacent to the tidal inlets and controls the inlet migration. Both of them are used to destabilize coastal systems and disrupt the natural sediment regimes.
