This is one of my biggest pet peeves....
Yes, sunblock loses it's effectiveness. No, they do not have an expiration date (arghhhh). However, be very careful when purchasing that 'new' bottle, especially from your local drugstore chain...I have noticed that they tend to have stock on the shelves that are in fact from last year!! EEK, this drives me nuts!!
I have often noticed that when the 'seasonal' shelf changes begin, especially in late winter/early spiring, those shelves start growing with old stock out of the stockrooms before new stuff is added!! For this reason I always look for new labeling on products, or something 'new' in sunblock, such as the 'colored' or 'foam' coopertone for kids (when they first came out), or whatever is 'new' for the year.
I really, really wish they were regulated with expiration dates.