Since kids can run hot or cold I think characters are actually more easy going with older guests. Anyone over 13 who wants to see Mickey is probably going to be in a fun mood vs. Young kids who can be shy/cranky/unsure/awestruck. Don't get me wrong - kids love characters.... but I get the feeling characters love kids-at-heart.
I loved the characters as a kid but my memorable interactions really started when I was a teenager. I was 'too cool' for Disney at 15 and some of the characters fed off of that - making faces or 'consoling' my mom. Captain hook made a big deal of inviting me to join his crew because he liked my style... and that was with me being a moody teen.
I did Disney for my college graduation present with my sister who was 18 at the time. We had a blast and some great times with the characters... they were animated and entertaining. When DH and I did CRT for our honeymoon, the princesses were great - asking him if he was Charming or not, comparing jobs of their princes with his... very personable. At Garden Grill, Chip and dale had a food fight with our rolls, Mickey tried to swap shirts with DH, and Pluto danced with me.
While I love seeing DD respond to the characters, I still enjoy the time they spend with me, too
DH also wants to add this: best interaction we saw was a group of young, droopy-pants, backwards hat wearing guys going nuts over Minnie. They waited (very patiently) to meet Mr and Mrs Mouse and acted like 7 year old when. They were next to her.... so funny and great to see