I think it takes in to account a variety of things. Child development, based on their own market research, I am sure comes in. They go off averages. Not all kids like the same things at the same age, and liking/noticing something isn't an indicator of readiness. My 5 year old loves to play Battleship. Does that mean he can? No, he's not developmentally ready to read a grid with out error. I don't take too much notice of ages on boxes, just use my own judgement if I think my child is ready or not.
I think they do a pretty good job. My kids tend to use things earlier than stated on boxes. Some labeling is done for their own protection- like anything that says not suitable for under 3 is more because the company doesn't want to get sued because a small child choked on a part.