Next year, Minnesota’s first baby boomers will cross a golden threshold — age 65 — and trigger an unprecedented surge in the state’s elderly population. The number of older Minnesotans will double in the next two decades, and for the first time pensioners will outnumber schoolchildren. The Twilight Zone, an occasional series by Star Tribune reporters, is examining the challenges of this “silver tsunami.’’ Today, a look at the rising pressure on caregivers.