with ongoing extracurriculars, we've found that when the age-grouping isn't working, it gets so obvious that someone (instructor/librarian/kid/us) speaks up.
it's happened over and over, but my best example involves dance. this fall, our girl (then 4) told us she wanted to quit dance, which was odd, considering she'd been begging to be allowed to take an extra ballet class all the prior year. i brought this up with the teacher, who said she'd like to move DD into a class with 7-8 year olds. i was a bit freaked out, and frankly so was the kid, but she went and tried the class - and it's been just right, even if she looks like a total munchkin beside those girls!
