I believe you've misunderstood. They're COOLING their house to 68 in the SUMMER. It sounds miserable to me no matter the season. In summer I'd bring a jacket and in winter I'd be bringing shorts and a t shirt.
No they're not. The OP wrote --
We keep our A/C set at 75 in the summer with have ceiling fans in most rooms, and 68 in the winter.
As to the OP's question, my DH is sensitive to heat and we keep our house at 74 much of the summer and rarely above 64 in the winter, except that we also use a wood stove so there is often a "warmer" spot in the house and sometimes the house itself is a good bit warmer (but we don't up the thermostat if it's not).
That said, I always yield to the requests of older generations when it comes to temperature settings for the reasons others have mentioned -- my understanding is that the older one gets, the less tolerant one is of temperature "extremes," where what constitutes extreme varies considerably by person/experience, and since mostly the people living in our house are, well, us, adjusting up or down for short periods of time to accommodate guests isn't a big deal, budget- or philosophy-wise.
And ... right. As many others have said, if you're willing to travel to my home to hang out and take care of my kid, I'll set the thermostat anywhere you want it. When my mom used to take care of my son our routine was that I'd meet her at a restaurant, treat her to breakfast, and then she'd take my son to her home so she could carry on with her usual routines ((as much as feasible while caring for him), and I'd come to her house to pick him up. Believe me, it was still a phenomenal bargain. Now that I'm providing care for another family member's young kid, I pretty much insist on the same (you get the kid to/from my house in a way that's convenient to me). Though come to think of it no one's treating me to breakfast, so maybe I should up my standards :) !