(Disclaimer, I have been guilty of this recently in reference to the 'blue-hairs', so I am adminishing myself as well)
We support the same team, we are on the same side.
Not every 'blue-hair' is an ignorant, anti-RR, 'down in front' type.
Not everyone under 25 is a newbie, drunk frat guy who spouts off without any knoeledge of past history and tradition.
Some people say stupid things, no matter what.
My suggestion in this case, instead of biting your tongue, politely inform said person that perhaps running up the score with starters on the field isn't the best coaching move and that seasoning some players on the depth chart, or trying a few new formations/looks will be more beneficial than an extra touchdown. We will likely need to depend on some of those players at some point this season. Aren't we glad we have some idea of what Floyd is capable of right now than having to drop him out there with no idea and get burned in the ND game?
Educate people, nicely, when they say stuff like this, if at all possible.