why is it harder as they're older?

I guess its easier to tell a 5 yr old "one more photo" they tend to enjoy the attention.  When you're surrounded by a ton of friends and family and the center of attention it's more embarrassing and I wouldn't want to cause that.  Although, I think I may show them more of the 'crappy' shots so they understand the few moments of embarrassment to get a good shot is by far a better choice than a bad shot spending eternity on a scrapbook page ;)