I've been to El Portus - I went in October a few years ago.
It was pretty quiet then but still plenty of sunshine during the day; the only problem that late in the season is that the sun dips behind the hills pretty early.
Hard to tell whether it gets much of a younger crowd in busier times, but that late in the season I was pretty much the only younger person. Not a problem, the people I met were all pretty young at heart and the bar was pretty sociable.
As well as the small but lovely beach, there's great walking and hiking right on the doorstep and you can scale the peak behind the campsite for a wonderful view of the site and coastline... just be careful as the path isn't that well marked.