All depends on the kid. There are 3-year-olds who can handle Disney, and 8-year-olds who can't. My girls could handle Disney pretty well at an early age, aside from normal stuff for that age(needing naps, etc.). But I've given up on taking them to Reds games and/or UK games that matter. Just doesn't hold their interest long enough, and we all end up frustrated.
You'll know when G is old enough to go. And I understand the "wait until they remember it" principle, but if you followed that to a T, we just wouldn't do ANYTHING with our kids until they're 8 or so. Sometimes it's cool just to see your kid happy in that moment, whether or not they'll remember it when they're 25. Again, I get it, especially from a financial perspective, but it's still cool to see your toddler interacting with their favorite character, even if they'll only have pics someday to remember it by.