Buy yourselves an autograph book, because the characters will spend more time to stop and chat with you while they are signing the book. That will give you (and them) more time to think of something to say.
I don't think anyone is too old for this unless they think they are. I have seen heart-warming situations where a senior rises from the table to hug the character, their face lit up, and they start to gush about how much they changed their lives when they were a kid...

It's magical.
As with age, there isn't a requirement to be nostalgic.
My fiance and I love the Disney character meals. I will share what happened to us. On his birthday, he requested a reservation at Cinderella's Royal Table in the castle for Lunch and 1900 Park Fare at the Grand Floridian for Dinner. These are fun restaurants, but this particular dinner was quite unusual.
It turned out that a group of 100+ cheerleaders canceled their dinner at Park Fare on our night--a freak occurrence according to our hostess... A total of 30 diners or less had the characters, Cinderella, Prince Charming, Anastasia, Drizella, Lady Tremaine (and the buffet) to ourselves the entire night! The characters came out to talk and play multiple times. At one point, all of the diners stood in the middle of the restaurant for group photo-ops, and with Anastasia and Drizella involved, that's like taking crazy family portraits.
Needless to say, we had the best time with the characters, and my fiance said that it was one of his most memorable birthdays. I've heard that the Mad Hatter is equally as funny and interactive as Anastasia and Drizella.
Have fun with the characters. You don't have to ask them about details relating to their movies if you don't want to. You can even just tell them something about yourself (e.g., your occupation, your hometown), or wear something interesting (e.g., perhaps with them on it), and they will run with it.