I dont know how things went down elsewhere, but in the theatre I saw the prequels in, same theatre each time, (even the same screen), the crowd opening night, (12:00am), was composed thusly:

(this is just a guestimate from surveying the crowd for 30-40 minutes before the show)

EP:1 75% 35 or over. Many, many folks in the 50+ range. 25% 35 or under. Almost no children. Very quiet audience. Smatterings of applause at the end. A handfull of costumes.

EP:2 90% 35 or under. 10% 35+. Much younger, more fun crowd. Very loud and excited. Nearly brought the house down when Yoda fired up his lightsaber! More costumes this time. More applause at the end this time around.

EP:3 50/50 old and young. Packed to the rafters craziness! I had the impression that, good or bad, all the 'old-timers' had showed up for one last romp. There were also lots and lots of children at this one, most wearing costumes. Costumes everywhere! Thunderous applause at the end.

Note the severe and sudden drop off of older audience members after EP1. I'd be interested to hear if this was the case where you saw the films.