I know 60 year old people who play pokemon TCG, so, honestly, as long as you enjoy something, you should never be ashamed of doing it because others think it's kiddish. --- You dont know it yet, but you're already dead. "Hokuto no Ken"
And particularly, rpg's/adventure games with a lot of grinding in them, which Monster Hunter 3 Ultimate may have.
I sort of feel guilty playing a game like this, that has hundreds of ours of gameplay, when I could be doing other stuff outside like throwing frisbees, going for walks, or running around in circles around a tree.
I sort of feel like 20+ years old gamers should stick to either playing really short games like arcade ones, or quite altogether, since we could be more productive doing other activities.