Companies such as Instacart, Uber and TaskRabbit may be known for their appeal to young, urban consumers, but they may soon influence older adults’ lives just as profoundly. Offering alternatives to traditional, senior-oriented services, these companies stand to transform how the older demographic gets things done. At 88, Sally Lindover already participates as both a user and a provider in what’s known as the on-demand and sharing economy. She recently signed up for home delivery…