Haha,
I doubt any tour would be full of 'young uns' to such an extent it would ruin your holiday. I am 26 and i find the vivacity and energy of the older (than me) JY'ers is greater than mine.
With regards to holidays, each person will have a different opinion but most think all the holidays are worth doing. I haven't done a long haul holiday yet, but have been to Croatia and france with JY and i can recommend both holidays to you. Istrian Coast was amazing and if you like Italy (which im sure you do), you will appreciate their influence.
Thanks
Mark