The age of a Flexible Flyer sled, specifically the Planet Jr No 251H model, can typically be traced back to the mid-20th century, around the 1950s or 1960s, as these sleds were popular during that era. The value of such a sled can vary greatly based on its condition, rarity, and collector interest, generally ranging from $50 to $200 or more. If it is in excellent condition with original paint and decals, it could fetch a higher price among collectors. For a precise valuation, it's advisable to consult vintage toy appraisal guides or auction listings.