I pardon them, but I don't condone it - I call my older relatives on what they are saying every time they say it and they have come to say those things less and less, even if it's only around me, I consider that a small victory. I don't ball them out for it, because I know they were raised in a way that accepted and even encouraged such opinions, but I do remind them that the fact doesn't excuse them for not having learned better in their long lifetimes. When someone is truly adamant about their -ism and refuses to listen to another side or makes it clear nothing will change their mind - old or not - I let them be and associate with them less for it.
My grandma makes an odd comment that is *slightly* -ist every now and then and I'll give her an eye roll and she'll shrug at me and say she's too old to change her mind and we just keep on, but if she says something extreme or blatantly offensive we'll have an argument about it. Small bits, I let go - big things, I feel I should speak up against. It really depends more on the size of the infraction than the age of any person for me.