Julie Verhoeff confesses she's not the kind of person who likes to exercise in a gym. But ask the 69-year-old to go square dancing, and she's ready to swing her partner and doh-si-doh.Julie Verhoeff and her 73-year-old husband, Hans Verhoeff, are among a growing number of seniors nationwide who have embraced square dancing as a way to both have fun and stay mentally alert and physically agile.The Grass Valley couple is a member of the Nevada City Goldancers, a 59-year-old square dancing club - the oldest of its kind in California, Julie Verhoeff said."Square dancing is all about listening, processing …