Why is it not OK for a 24 year old to walk into a liquor store with somebody that has an ID to buy alcohol? ?

It is ok, at least in Canada I've never had a problem. I've always gone in with my dad even when I was younger and still to this day, once in a while. No one has ever said anything to me as far as I can remember. I would always carry the stuff for him too, and no one would bat an eye. Plus, I've seen many other people in there with their kids. Maybe it's different depending on where you are but, as far as I'm aware, as long as you aren't actually purchasing anything and you're there with someone of legal age, there isn't, or shouldn't be, a problem. The 24 year old is also of age, so there should literally be zero problems at all. Also, again as far as I'm aware, that person doesn't need to show anyone valid ID when asked either because they are not purchasing anything. I have a feeling some of these employees just love the power trip, though that may not be the case for each individual store. I'm just going based off of my own experiences. 

Alcohol and children do not mix.

I don't know, why is it “not ok” to do that?

You're the one suggesting it's not

It is legal so yes.As long as both people are of age.

It's not how old you are the law says that you must have identification to be there! It would be just as wrong for a 60-year-old man to accompany a 50-year-old man to use his identification it's the law no identification you're not even supposed to be there

Maybe the 24 year old looks to be underage.