I too grew up on machines that were mid-low range and was constantly asking more of them than they could handle, so I learned to stomach pretty miserable FPS. In the end though it’s highly context sensitive - the less movement (and in particular camera movement) the game has the lower the frame rate you can get away with.

As a general rule I would say 25 FPS is the absolute lower limit, but around 40 is probably more in line with your “this is fine and I’m going to have a great time” definition. However, for something like a fast paced shooter it’s more like 60 FPS minimum.