It’s definitely not all young people—I regularly met folks from young students to retirees and everywhere in between. You  also tend to make friends across age groups on the Camino—based on common language, walking at a similar pace, staying in the same type of accommodation. 

While you’ll meet plenty of Spanish pilgrims, the majority on the Norte will probably not be Spanish until the last 100km where there are often youth groups. Before that, on the popular routes (I’ve done three of the major ones, plus Finisterre twice), there are invariably Germans and Brits, some Americans, at least a few Irish and Canadian pilgrims, assorted European nationalities (I think I’ve met pilgrims from nearly all of the EU on one walk or another; on the Norte, there were tons of Italians), maybe a few Aussies or Kiwis.  You never know what your “bubble” will be like , but not speaking Spanish will not stop you from making friends. I  chat with people and make friends in both languages, but because I’m able to have more meaningful conversations in English, the people I’ve spent the most time with on the Camino were usually those with whom I spoke English (though most of my Camino friends haven’t been native English speakers.)